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Thursday, August 19, 2010
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SQL SERVER – 2008 – Interview Questions and
SQL SERVER – 2008 – Interview Questions
1) General Questions of SQL SERVER
What is RDBMS?
Relational Data Base Management Systems (RDBMS) are database management systems that maintain data records and indices in tables. Relationships may be created and maintained across and among the data and tables. In a relational database, relationships between data items are expressed by means of tables. Interdependencies among these tables are expressed by data values rather than by pointers. This allows a high degree of data independence. An RDBMS has the capability to recombine the data items from different files, providing powerful tools for data usage. (Read More Here)
What are the properties of the Relational tables?
Relational tables have six properties:
• Values are atomic.
• Column values are of the same kind.
• Each row is unique.
• The sequence of columns is insignificant.
• The sequence of rows is insignificant.
• Each column must have a unique name.
What is Normalization?
Database normalization is a data design and organization process applied to data structures based on rules that help building relational databases. In relational database design, the process of organizing data to minimize redundancy is called normalization. Normalization usually involves dividing a database into two or more tables and defining relationships between the tables. The objective is to isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships.
What are different normalization forms?
1NF: Eliminate Repeating Groups
Make a separate table for each set of related attributes, and give each table a primary key. Each field contains at most one value from its attribute domain.
2NF: Eliminate Redundant Data
If an attribute depends on only part of a multi-valued key, remove it to a separate table.
3NF: Eliminate Columns Not Dependent On Key
If attributes do not contribute to a description of the key, remove them to a separate table. All attributes must be directly dependent on the primary key. (Read More Here)
BCNF: Boyce-Codd Normal Form
If there are non-trivial dependencies between candidate key attributes, separate them out into distinct tables.
4NF: Isolate Independent Multiple Relationships
No table may contain two or more 1:n or n:m relationships that are not directly related.
5NF: Isolate Semantically Related Multiple Relationships
There may be practical constrains on information that justify separating logically related many-to-many relationships.
ONF: Optimal Normal Form
A model limited to only simple (elemental) facts, as expressed in Object Role Model notation.
DKNF: Domain-Key Normal Form
A model free from all modification anomalies is said to be in DKNF.
Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it must first fulfill all the criteria of a 2NF and 1NF database.
What is De-normalization?
De-normalization is the process of attempting to optimize the performance of a database by adding redundant data. It is sometimes necessary because current DBMSs implement the relational model poorly. A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De-normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access.
What is Stored Procedure?
A stored procedure is a named group of SQL statements that have been previously created and stored in the server database. Stored procedures accept input parameters so that a single procedure can be used over the network by several clients using different input data. And when the procedure is modified, all clients automatically get the new version. Stored procedures reduce network traffic and improve performance. Stored procedures can be used to help ensure the integrity of the database.
e.g. sp_helpdb, sp_renamedb, sp_depends etc.
What is Trigger?
A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or UPDATE) occurs. Triggers are stored in and managed by the DBMS. Triggers are used to maintain the referential integrity of data by changing the data in a systematic fashion. A trigger cannot be called or executed; DBMS automatically fires the trigger as a result of a data modification to the associated table. Triggers can be viewed as similar to stored procedures in that both consist of procedural logic that is stored at the database level. Stored procedures, however, are not event-drive and are not attached to a specific table as triggers are. Stored procedures are explicitly executed by invoking a CALL to the procedure while triggers are implicitly executed. In addition, triggers can also execute stored procedures.
Nested Trigger: A trigger can also contain INSERT, UPDATE and DELETE logic within itself, so when the trigger is fired because of data modification it can also cause another data modification, thereby firing another trigger. A trigger that contains data modification logic within itself is called a nested trigger. (Read More Here)
What is View?
A simple view can be thought of as a subset of a table. It can be used for retrieving data, as well as updating or deleting rows. Rows updated or deleted in the view are updated or deleted in the table the view was created with. It should also be noted that as data in the original table changes, so does data in the view, as views are the way to look at part of the original table. The results of using a view are not permanently stored in the database. The data accessed through a view is actually constructed using standard T-SQL select command and can come from one to many different base tables or even other views.
What is Index?
An index is a physical structure containing pointers to the data. Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes; they are just used to speed up queries. Effective indexes are one of the best ways to improve performance in a database application. A table scan happens when there is no index available to help a query. In a table scan SQL Server examines every row in the table to satisfy the query results. Table scans are sometimes unavoidable, but on large tables, scans have a terrific impact on performance.
What is a Linked Server?
Linked Servers is a concept in SQL Server by which we can add other SQL Server to a Group and query both the SQL Server dbs using T-SQL Statements. With a linked server, you can create very clean, easy to follow, SQL statements that allow remote data to be retrieved, joined and combined with local data. Stored Procedure sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new Linked Server. (Read More Here)
SQL SERVER – 2008 – Interview Questions and Answers – Part 2
1) General Questions of SQL SERVER
What is Cursor?
Cursor is a database object used by applications to manipulate data in a set on a row-by-row basis, instead of the typical SQL commands that operate on all the rows in the set at one time.
In order to work with a cursor we need to perform some steps in the following order:
• Declare cursor
• Open cursor
• Fetch row from the cursor
• Process fetched row
• Close cursor
• Deallocate cursor (Read More Here)
What is Collation?
Collation refers to a set of rules that determine how data is sorted and compared. Character data is sorted using rules that define the correct character sequence, with options for specifying case sensitivity, accent marks, kana character types and character width. (Read More Here)
What is Difference between Function and Stored Procedure?
UDF can be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section where as Stored procedures cannot be. UDFs that return tables can be treated as another rowset. This can be used in JOINs with other tables. Inline UDF’s can be thought of as views that take parameters and can be used in JOINs and other Rowset operations.
What is sub-query? Explain properties of sub-query?
Sub-queries are often referred to as sub-selects, as they allow a SELECT statement to be executed arbitrarily within the body of another SQL statement. A sub-query is executed by enclosing it in a set of parentheses. Sub-queries are generally used to return a single row as an atomic value, though they may be used to compare values against multiple rows with the IN keyword.
A subquery is a SELECT statement that is nested within another T-SQL statement. A subquery SELECT statement if executed independently of the T-SQL statement, in which it is nested, will return a resultset. Meaning a subquery SELECT statement can standalone and is not depended on the statement in which it is nested. A subquery SELECT statement can return any number of values, and can be found in, the column list of a SELECT statement, a FROM, GROUP BY, HAVING, and/or ORDER BY clauses of a T-SQL statement. A Subquery can also be used as a parameter to a function call. Basically a subquery can be used anywhere an expression can be used. (Read More Here)
What are different Types of Join?
Cross Join
A cross join that does not have a WHERE clause produces the Cartesian product of the tables involved in the join. The size of a Cartesian product result set is the number of rows in the first table multiplied by the number of rows in the second table. The common example is when company wants to combine each product with a pricing table to analyze each product at each price.
Inner Join
A join that displays only the rows that have a match in both joined tables is known as inner Join. This is the default type of join in the Query and View Designer.
Outer Join
A join that includes rows even if they do not have related rows in the joined table is an Outer Join. You can create three different outer join to specify the unmatched rows to be included:
• Left Outer Join: In Left Outer Join all rows in the first-named table i.e. “left” table, which appears leftmost in the JOIN clause are included. Unmatched rows in the right table do not appear.
• Right Outer Join: In Right Outer Join all rows in the second-named table i.e. “right” table, which appears rightmost in the JOIN clause are included. Unmatched rows in the left table are not included.
• Full Outer Join: In Full Outer Join all rows in all joined tables are included, whether they are matched or not.
Self Join
This is a particular case when one table joins to itself, with one or two aliases to avoid confusion. A self join can be of any type, as long as the joined tables are the same. A self join is rather unique in that it involves a relationship with only one table. The common example is when company has a hierarchal reporting structure whereby one member of staff reports to another. Self Join can be Outer Join or Inner Join. (Read More Here)
What are primary keys and foreign keys?
Primary keys are the unique identifiers for each row. They must contain unique values and cannot be null. Due to their importance in relational databases, Primary keys are the most fundamental of all keys and constraints. A table can have only one Primary key.
Foreign keys are both a method of ensuring data integrity and a manifestation of the relationship between tables.
What is User Defined Functions? What kind of User-Defined Functions can be created?
User-Defined Functions allow defining its own T-SQL functions that can accept 0 or more parameters and return a single scalar data value or a table data type.
Different Kinds of User-Defined Functions created are:
Scalar User-Defined Function
A Scalar user-defined function returns one of the scalar data types. Text, ntext, image and timestamp data types are not supported. These are the type of user-defined functions that most developers are used to in other programming languages. You pass in 0 to many parameters and you get a return value.
Inline Table-Value User-Defined Function
An Inline Table-Value user-defined function returns a table data type and is an exceptional alternative to a view as the user-defined function can pass parameters into a T-SQL select command and in essence provide us with a parameterized, non-updateable view of the underlying tables.
Multi-statement Table-Value User-Defined Function
A Multi-Statement Table-Value user-defined function returns a table and is also an exceptional alternative to a view as the function can support multiple T-SQL statements to build the final result where the view is limited to a single SELECT statement. Also, the ability to pass parameters into a TSQL select command or a group of them gives us the capability to in essence create a parameterized, non-updateable view of the data in the underlying tables. Within the create function command you must define the table structure that is being returned. After creating this type of user-defined function, It can be used in the FROM clause of a T-SQL command unlike the behavior found when using a stored procedure which can also return record sets. (Read Here For Example)
What is Identity?
Identity (or AutoNumber) is a column that automatically generates numeric values. A start and increment value can be set, but most DBA leave these at 1. A GUID column also generates numbers; the value of this cannot be controlled. Identity/GUID columns do not need to be indexed.
What is DataWarehousing?
• Subject-oriented, meaning that the data in the database is organized so that all the data elements relating to the same real-world event or object are linked together;
• Time-variant, meaning that the changes to the data in the database are tracked and recorded so that reports can be produced showing changes over time;
• Non-volatile, meaning that data in the database is never over-written or deleted, once committed, the data is static, read-only, but retained for future reporting.
• Integrated, meaning that the database contains data from most or all of an organization’s operational applications, and that this data is made consistent.
SQL SERVER – 2008 – Interview Questions and Answers – Part 3
1) General Questions of SQL SERVER
2) Common Questions Asked
Which TCP/IP port does SQL Server run on? How can it be changed?
SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties -> Port number, both on client and the server.
What are the difference between clustered and a non-clustered index? (Read More Here)
A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.
A non clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.
What are the different index configurations a table can have?
A table can have one of the following index configurations:
• No indexes
• A clustered index
• A clustered index and many nonclustered indexes
• A nonclustered index
• Many nonclustered indexes
What are different types of Collation Sensitivity?
Case sensitivity – A and a, B and b, etc.
Accent sensitivity – a and á, o and ó, etc.
Kana Sensitivity – When Japanese kana characters Hiragana and Katakana are treated differently, it is called Kana sensitive.
Width sensitivity – A single-byte character (half-width) and the same character represented as a double-byte character (full-width) are treated differently than it is width sensitive. (Read More Here)
What is OLTP (Online Transaction Processing)?
In OLTP – online transaction processing systems relational database design use the discipline of data modeling and generally follow the Codd rules of data normalization in order to ensure absolute data integrity. Using these rules complex information is broken down into its most simple structures (a table) where all of the individual atomic level elements relate to each other and satisfy the normalization rules.
What’s the difference between a primary key and a unique key?
Both primary key and unique key enforces uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn’t allow NULLs, but unique key allows one NULL only. (Read More Here)
What is difference between DELETE & TRUNCATE commands?
Delete command removes the rows from a table based on the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table and there will be no data in the table after we run the truncate command.
TRUNCATE
• TRUNCATE is faster and uses fewer system and transaction log resources than DELETE.
• TRUNCATE removes the data by deallocating the data pages used to store the table’s data, and only the page deallocations are recorded in the transaction log.
• TRUNCATE removes all rows from a table, but the table structure, its columns, constraints, indexes and so on, remains. The counter used by an identity for new rows is reset to the seed for the column.
• You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint. Because TRUNCATE TABLE is not logged, it cannot activate a trigger.
• TRUNCATE cannot be rolled back.
• TRUNCATE is DDL Command.
• TRUNCATE Resets identity of the table
DELETE
• DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.
• If you want to retain the identity counter, use DELETE instead. If you want to remove table definition and its data, use the DROP TABLE statement.
• DELETE Can be used with or without a WHERE clause
• DELETE Activates Triggers.
• DELETE can be rolled back.
• DELETE is DML Command.
• DELETE does not reset identity of the table.
(Read More Here)
When is the use of UPDATE_STATISTICS command?
This command is basically used when a large processing of data has occurred. If a large amount of deletions any modification or Bulk Copy into the tables has occurred, it has to update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.
What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
They specify a search condition for a group or an aggregate. But the difference is that HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query whereas WHERE Clause is applied to each row before they are part of the GROUP BY function in a query. (Read More Here)
What are the properties and different Types of Sub-Queries?
Properties of Sub-Query
• A sub-query must be enclosed in the parenthesis.
• A sub-query must be put in the right hand of the comparison operator, and
• A sub-query cannot contain an ORDER-BY clause.
• A query can contain more than one sub-query.
Types of Sub-query
• Single-row sub-query, where the sub-query returns only one row.
• Multiple-row sub-query, where the sub-query returns multiple rows,. and
• Multiple column sub-query, where the sub-query returns multiple columns
What is SQL Profiler?
SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance of Microsoft SQL Server. You can capture and save data about each event to a file or SQL Server table to analyze later. For example, you can monitor a production environment to see which stored procedures are hampering performances by executing too slowly.
Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming too large, you can filter them based on the information you want, so that only a subset of the event data is collected. Monitoring too many events adds overhead to the server and the monitoring process and can cause the trace file or trace table to grow very large, especially when the monitoring process takes place over a long period of time.
What are the authentication modes in SQL Server? How can it be changed?
Windows mode and Mixed Mode – SQL & Windows.
To change authentication mode in SQL Server click Start, Programs, Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group. Select the server then from the Tools menu select SQL Server Configuration Properties, and choose the Security page.
SQL SERVER – 2008 – Interview Questions and Answers – Part 4
1) General Questions of SQL SERVER
Which command using Query Analyzer will give you the version of SQL server and operating system?
SELECT SERVERPROPERTY ('productversion'), SERVERPROPERTY ('productlevel'), SERVERPROPERTY ('edition')
What is SQL Server Agent?
SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the implementation of tasks for the DBA, with its full-function scheduling engine, which allows you to schedule your own jobs and scripts. (Read More Here)
Can a stored procedure call itself or recursive stored procedure? How much level SP nesting is possible?
Yes. Because Transact-SQL supports recursion, you can write stored procedures that call themselves. Recursion can be defined as a method of problem solving wherein the solution is arrived at by repetitively applying it to subsets of the problem. A common application of recursive logic is to perform numeric computations that lend themselves to repetitive evaluation by the same processing steps. Stored procedures are nested when one stored procedure calls another or executes managed code by referencing a CLR routine, type, or aggregate. You can nest stored procedures and managed code references up to 32 levels.
What is Log Shipping?
Log shipping is the process of automating the backup of database and transaction log files on a production SQL server, and then restoring them onto a standby server. Enterprise Editions only supports log shipping. In log shipping the transactional log file from one server is automatically updated into the backup database on the other server. If one server fails, the other server will have the same db and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs throughout the day and automatically restore them on the standby server at defined interval.
Name 3 ways to get an accurate count of the number of records in a table?
SELECT * FROM table1
SELECT COUNT(*) FROM table1
SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table1) AND indid < 2
What does it mean to have QUOTED_IDENTIFIER ON? What are the implications of having it OFF?
When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation marks, and literals must be delimited by single quotation marks. When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all Transact-SQL rules for identifiers. (Read More Here)
What is the difference between a Local and a Global temporary table?
A local temporary table exists only for the duration of a connection or, if defined inside a compound statement, for the duration of the compound statement.
A global temporary table remains in the database permanently, but the rows exist only within a given connection. When connection is closed, the data in the global temporary table disappears. However, the table definition remains with the database for access when database is opened next time.
What is the STUFF function and how does it differ from the REPLACE function?
STUFF function is used to overwrite existing characters. Using this syntax, STUFF (string_expression, start, length, replacement_characters), string_expression is the string that will have characters substituted, start is the starting position, length is the number of characters in the string that are substituted, and replacement_characters are the new characters interjected into the string. REPLACE function to replace existing characters of all occurrences. Using the syntax REPLACE (string_expression, search_string, replacement_string), where every incidence of search_string found in the string_expression will be replaced with replacement_string.
What is PRIMARY KEY?
A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should have a primary key constraint to uniquely identify each row and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.
What is UNIQUE KEY constraint?
A UNIQUE constraint enforces the uniqueness of the values in a set of columns, so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.
What is FOREIGN KEY?
A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.
What is CHECK Constraint?
A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity. (Read More Here)
What is NOT NULL Constraint?
A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints.
(Read More Here)
How to get @@ERROR and @@ROWCOUNT at the same time?
If @@Rowcount is checked after Error checking statement then it will have 0 as the value of @@Recordcount as it would have been reset. And if @@Recordcount is checked before the error-checking statement then @@Error would get reset. To get @@error and @@rowcount at the same time do both in same statement and store them in local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR
What is a Scheduled Jobs or What is a Scheduled Tasks?
Scheduled tasks let user automate processes that run on regular or predictable cycles. User can schedule administrative tasks, such as cube processing, to run during times of slow business activity. User can also determine the order in which tasks run by creating job steps within a SQL Server Agent job. E.g. back up database, Update Stats of Tables. Job steps give user control over flow of execution. If one job fails, user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.
What are the advantages of using Stored Procedures?
• Stored procedure can reduced network traffic and latency, boosting application performance.
• Stored procedure execution plans can be reused, staying cached in SQL Server’s memory, reducing server overhead.
• Stored procedures help promote code reuse.
• Stored procedures can encapsulate logic. You can change stored procedure code without affecting clients.
• Stored procedures provide better security to your data.
What is a table called, if it has neither Cluster nor Non-cluster Index? What is it used for?
Unindexed table or Heap. Microsoft Press Books and Book on Line (BOL) refers it as Heap. A heap is a table that does not have a clustered index and, therefore, the pages are not linked by pointers. The IAM pages are the only structures that link the pages in a table together. Unindexed tables are good for fast storing of data. Many times it is better to drop all indexes from table and then do bulk of inserts and to restore those indexes after that.
Can SQL Servers linked to other servers like Oracle?
SQL Server can be linked to any server provided it has OLE-DB provider from Microsoft to allow a link. E.g. Oracle has an OLE-DB provider for oracle that Microsoft provides to add it as linked server to SQL Server group
What is BCP? When does it used?
BulkCopy is a tool used to copy huge amount of data from tables and views. BCP does not copy the structures same as source to destination. BULK INSERT command helps to import a data file into a database table or view in a user-specified format.
SQL SERVER – 2008 – Interview Questions and Answers – Part 5
What command do we use to rename a db, a table and a column?
To rename db
sp_renamedb 'oldname' , 'newname'
If someone is using db it will not accept sp_renmaedb. In that case first bring db to single user using sp_dboptions. Use sp_renamedb to rename database. Use sp_dboptions to bring database to multi user mode.
E.g.
USE master;
GO
EXEC sp_dboption AdventureWorks, 'Single User', True
GO
EXEC sp_renamedb 'AdventureWorks', 'AdventureWorks_New'
GO
EXEC sp_dboption AdventureWorks, 'Single User', False
GO
To rename Table
We can change the table name using sp_rename as follows,
sp_rename 'oldTableName' 'newTableName'
E.g.
sp_RENAME 'Table_First', 'Table_Last'
GO
To rename Column
The script for renaming any column :
sp_rename 'TableName.[OldcolumnName]', 'NewColumnName', 'Column'
E.g.
sp_RENAME 'Table_First.Name', 'NameChange' , 'COLUMN'
GO
What are sp_configure commands and set commands?
Use sp_configure to display or change server-level settings. To change database-level settings, use ALTER DATABASE. To change settings that affect only the current user session, use the SET statement.
E.g.
sp_CONFIGURE 'show advanced', 0
GO
RECONFIGURE
GO
sp_CONFIGURE
GO
You can run following command and check advance global configuration settings.
sp_CONFIGURE 'show advanced', 1
GO
RECONFIGURE
GO
sp_CONFIGURE
GO
(Read More Here)
How to implement one-to-one, one-to-many and many-to-many relationships while designing tables?
One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships.
Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table.
What is an execution plan? When would you use it? How would you view the execution plan?
An execution plan is basically a road map that graphically or textually shows the data retrieval methods chosen by the SQL Server query optimizer for a stored procedure or ad-hoc query and is a very useful tool for a developer to understand the performance characteristics of a query or stored procedure since the plan is the one that SQL Server will place in its cache and use to execute the stored procedure or query. From within Query Analyzer is an option called “Show Execution Plan” (located on the Query drop-down menu). If this option is turned on it will display query execution plan in separate window when query is ran again.
SQL SERVER – 2008 – Interview Questions and Answers – Part 7
How can we rewrite sub-queries into simple select statements or with joins?
Yes we can write using Common Table Expression (CTE). A Common Table Expression (CTE) is an expression that can be thought of as a temporary result set which is defined within the execution of a single SQL statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query.
E.g.
USE AdventureWorks
GO
WITH EmployeeDepartment_CTE AS (
SELECT EmployeeID,DepartmentID,ShiftID
FROM HumanResources.EmployeeDepartmentHistory
)
SELECT ecte.EmployeeId,ed.DepartmentID, ed.Name,ecte.ShiftID
FROM HumanResources.Department ed
INNER JOIN EmployeeDepartment_CTE ecte ON ecte.DepartmentID = ed.DepartmentID
GO
What is CLR?
In SQL Server 2008, SQL Server objects such as user-defined functions can be created using such CLR languages. This CLR language support extends not only to user-defined functions, but also to stored procedures and triggers. You can develop such CLR add-ons to SQL Server using Visual Studio 2008. (Read More Here)
What are synonyms?
Synonyms give you the ability to provide alternate names for database objects. You can alias object names; for example, using the Employee table as Emp. You can also shorten names. This is especially useful when dealing with three and four part names; for example, shortening server.database.owner.object to object. (Read More Here)
What is LINQ?
Language Integrated Query (LINQ) adds the ability to query objects using .NET languages. The LINQ to SQL object/relational mapping (O/RM) framework provides the following basic features:
• Tools to create classes (usually called entities) mapped to database tables
• Compatibility with LINQ’s standard query operations
• The DataContext class, with features such as entity record monitoring, automatic SQL statement generation, record concurrency detection, and much more
What is Isolation Levels?
Transactions specify an isolation level that defines the degree to which one transaction must be isolated from resource or data modifications made by other transactions. Isolation levels are described in terms of which concurrency side-effects, such as dirty reads or phantom reads, are allowed.
Transaction isolation levels control:
• Whether locks are taken when data is read, and what type of locks are requested.
• How long the read locks are held.
• Whether a read operation referencing rows modified by another transaction:
• Blocks until the exclusive lock on the row is freed.
• Retrieves the committed version of the row that existed at the time the statement or transaction started.
• Reads the uncommitted data modification. (Read More Here)
What is use of EXCEPT Clause?
EXCEPT clause is similar to MINUS operation in Oracle. The EXCEPT query and MINUS query returns all rows in the first query that are not returned in the second query. Each SQL statement within the EXCEPT query and MINUS query must have the same number of fields in the result sets with similar data types. (Read More Here)
What is XPath?
XPath uses a set of expressions to select nodes to be processed. The most common expression that you’ll use is the location path expression, which returns back a set of nodes called a node set. XPath can use both an unabbreviated and an abbreviated syntax. The following is the unabbreviated syntax for a location path:
/axisName::nodeTest[predicate]/axisName::nodeTest[predicate]
What is NOLOCK?
Using the NOLOCK query optimizer hint is generally considered good practice in order to improve concurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks are taken when data is read. The result is a Dirty Read, which means that another process could be updating the data at the exact time you are reading it. There are no guarantees that your query will retrieve the most recent data. The advantage to performance is that your reading of data will not block updates from taking place, and updates will not block your reading of data. SELECT statements take Shared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, but other processes are blocked from modifying the data. The updates will queue until all the reads have completed, and reads requested after the update will wait for the updates to complete. The result to your system is delay (blocking). (Read More Here)
How would you handle error in SQL SERVER 2008?
SQL Server now supports the use of TRY…CATCH constructs for providing rich error handling. TRY…CATCH lets us build error handling at the level we need, in the way we need to, by setting a region where if any error occurs, it will break out of the region and head to an error handler. The basic structure is as follows:
BEGIN TRY
1) General Questions of SQL SERVER
What is RDBMS?
Relational Data Base Management Systems (RDBMS) are database management systems that maintain data records and indices in tables. Relationships may be created and maintained across and among the data and tables. In a relational database, relationships between data items are expressed by means of tables. Interdependencies among these tables are expressed by data values rather than by pointers. This allows a high degree of data independence. An RDBMS has the capability to recombine the data items from different files, providing powerful tools for data usage. (Read More Here)
What are the properties of the Relational tables?
Relational tables have six properties:
• Values are atomic.
• Column values are of the same kind.
• Each row is unique.
• The sequence of columns is insignificant.
• The sequence of rows is insignificant.
• Each column must have a unique name.
What is Normalization?
Database normalization is a data design and organization process applied to data structures based on rules that help building relational databases. In relational database design, the process of organizing data to minimize redundancy is called normalization. Normalization usually involves dividing a database into two or more tables and defining relationships between the tables. The objective is to isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships.
What are different normalization forms?
1NF: Eliminate Repeating Groups
Make a separate table for each set of related attributes, and give each table a primary key. Each field contains at most one value from its attribute domain.
2NF: Eliminate Redundant Data
If an attribute depends on only part of a multi-valued key, remove it to a separate table.
3NF: Eliminate Columns Not Dependent On Key
If attributes do not contribute to a description of the key, remove them to a separate table. All attributes must be directly dependent on the primary key. (Read More Here)
BCNF: Boyce-Codd Normal Form
If there are non-trivial dependencies between candidate key attributes, separate them out into distinct tables.
4NF: Isolate Independent Multiple Relationships
No table may contain two or more 1:n or n:m relationships that are not directly related.
5NF: Isolate Semantically Related Multiple Relationships
There may be practical constrains on information that justify separating logically related many-to-many relationships.
ONF: Optimal Normal Form
A model limited to only simple (elemental) facts, as expressed in Object Role Model notation.
DKNF: Domain-Key Normal Form
A model free from all modification anomalies is said to be in DKNF.
Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it must first fulfill all the criteria of a 2NF and 1NF database.
What is De-normalization?
De-normalization is the process of attempting to optimize the performance of a database by adding redundant data. It is sometimes necessary because current DBMSs implement the relational model poorly. A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De-normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access.
What is Stored Procedure?
A stored procedure is a named group of SQL statements that have been previously created and stored in the server database. Stored procedures accept input parameters so that a single procedure can be used over the network by several clients using different input data. And when the procedure is modified, all clients automatically get the new version. Stored procedures reduce network traffic and improve performance. Stored procedures can be used to help ensure the integrity of the database.
e.g. sp_helpdb, sp_renamedb, sp_depends etc.
What is Trigger?
A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or UPDATE) occurs. Triggers are stored in and managed by the DBMS. Triggers are used to maintain the referential integrity of data by changing the data in a systematic fashion. A trigger cannot be called or executed; DBMS automatically fires the trigger as a result of a data modification to the associated table. Triggers can be viewed as similar to stored procedures in that both consist of procedural logic that is stored at the database level. Stored procedures, however, are not event-drive and are not attached to a specific table as triggers are. Stored procedures are explicitly executed by invoking a CALL to the procedure while triggers are implicitly executed. In addition, triggers can also execute stored procedures.
Nested Trigger: A trigger can also contain INSERT, UPDATE and DELETE logic within itself, so when the trigger is fired because of data modification it can also cause another data modification, thereby firing another trigger. A trigger that contains data modification logic within itself is called a nested trigger. (Read More Here)
What is View?
A simple view can be thought of as a subset of a table. It can be used for retrieving data, as well as updating or deleting rows. Rows updated or deleted in the view are updated or deleted in the table the view was created with. It should also be noted that as data in the original table changes, so does data in the view, as views are the way to look at part of the original table. The results of using a view are not permanently stored in the database. The data accessed through a view is actually constructed using standard T-SQL select command and can come from one to many different base tables or even other views.
What is Index?
An index is a physical structure containing pointers to the data. Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes; they are just used to speed up queries. Effective indexes are one of the best ways to improve performance in a database application. A table scan happens when there is no index available to help a query. In a table scan SQL Server examines every row in the table to satisfy the query results. Table scans are sometimes unavoidable, but on large tables, scans have a terrific impact on performance.
What is a Linked Server?
Linked Servers is a concept in SQL Server by which we can add other SQL Server to a Group and query both the SQL Server dbs using T-SQL Statements. With a linked server, you can create very clean, easy to follow, SQL statements that allow remote data to be retrieved, joined and combined with local data. Stored Procedure sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new Linked Server. (Read More Here)
SQL SERVER – 2008 – Interview Questions and Answers – Part 2
1) General Questions of SQL SERVER
What is Cursor?
Cursor is a database object used by applications to manipulate data in a set on a row-by-row basis, instead of the typical SQL commands that operate on all the rows in the set at one time.
In order to work with a cursor we need to perform some steps in the following order:
• Declare cursor
• Open cursor
• Fetch row from the cursor
• Process fetched row
• Close cursor
• Deallocate cursor (Read More Here)
What is Collation?
Collation refers to a set of rules that determine how data is sorted and compared. Character data is sorted using rules that define the correct character sequence, with options for specifying case sensitivity, accent marks, kana character types and character width. (Read More Here)
What is Difference between Function and Stored Procedure?
UDF can be used in the SQL statements anywhere in the WHERE/HAVING/SELECT section where as Stored procedures cannot be. UDFs that return tables can be treated as another rowset. This can be used in JOINs with other tables. Inline UDF’s can be thought of as views that take parameters and can be used in JOINs and other Rowset operations.
What is sub-query? Explain properties of sub-query?
Sub-queries are often referred to as sub-selects, as they allow a SELECT statement to be executed arbitrarily within the body of another SQL statement. A sub-query is executed by enclosing it in a set of parentheses. Sub-queries are generally used to return a single row as an atomic value, though they may be used to compare values against multiple rows with the IN keyword.
A subquery is a SELECT statement that is nested within another T-SQL statement. A subquery SELECT statement if executed independently of the T-SQL statement, in which it is nested, will return a resultset. Meaning a subquery SELECT statement can standalone and is not depended on the statement in which it is nested. A subquery SELECT statement can return any number of values, and can be found in, the column list of a SELECT statement, a FROM, GROUP BY, HAVING, and/or ORDER BY clauses of a T-SQL statement. A Subquery can also be used as a parameter to a function call. Basically a subquery can be used anywhere an expression can be used. (Read More Here)
What are different Types of Join?
Cross Join
A cross join that does not have a WHERE clause produces the Cartesian product of the tables involved in the join. The size of a Cartesian product result set is the number of rows in the first table multiplied by the number of rows in the second table. The common example is when company wants to combine each product with a pricing table to analyze each product at each price.
Inner Join
A join that displays only the rows that have a match in both joined tables is known as inner Join. This is the default type of join in the Query and View Designer.
Outer Join
A join that includes rows even if they do not have related rows in the joined table is an Outer Join. You can create three different outer join to specify the unmatched rows to be included:
• Left Outer Join: In Left Outer Join all rows in the first-named table i.e. “left” table, which appears leftmost in the JOIN clause are included. Unmatched rows in the right table do not appear.
• Right Outer Join: In Right Outer Join all rows in the second-named table i.e. “right” table, which appears rightmost in the JOIN clause are included. Unmatched rows in the left table are not included.
• Full Outer Join: In Full Outer Join all rows in all joined tables are included, whether they are matched or not.
Self Join
This is a particular case when one table joins to itself, with one or two aliases to avoid confusion. A self join can be of any type, as long as the joined tables are the same. A self join is rather unique in that it involves a relationship with only one table. The common example is when company has a hierarchal reporting structure whereby one member of staff reports to another. Self Join can be Outer Join or Inner Join. (Read More Here)
What are primary keys and foreign keys?
Primary keys are the unique identifiers for each row. They must contain unique values and cannot be null. Due to their importance in relational databases, Primary keys are the most fundamental of all keys and constraints. A table can have only one Primary key.
Foreign keys are both a method of ensuring data integrity and a manifestation of the relationship between tables.
What is User Defined Functions? What kind of User-Defined Functions can be created?
User-Defined Functions allow defining its own T-SQL functions that can accept 0 or more parameters and return a single scalar data value or a table data type.
Different Kinds of User-Defined Functions created are:
Scalar User-Defined Function
A Scalar user-defined function returns one of the scalar data types. Text, ntext, image and timestamp data types are not supported. These are the type of user-defined functions that most developers are used to in other programming languages. You pass in 0 to many parameters and you get a return value.
Inline Table-Value User-Defined Function
An Inline Table-Value user-defined function returns a table data type and is an exceptional alternative to a view as the user-defined function can pass parameters into a T-SQL select command and in essence provide us with a parameterized, non-updateable view of the underlying tables.
Multi-statement Table-Value User-Defined Function
A Multi-Statement Table-Value user-defined function returns a table and is also an exceptional alternative to a view as the function can support multiple T-SQL statements to build the final result where the view is limited to a single SELECT statement. Also, the ability to pass parameters into a TSQL select command or a group of them gives us the capability to in essence create a parameterized, non-updateable view of the data in the underlying tables. Within the create function command you must define the table structure that is being returned. After creating this type of user-defined function, It can be used in the FROM clause of a T-SQL command unlike the behavior found when using a stored procedure which can also return record sets. (Read Here For Example)
What is Identity?
Identity (or AutoNumber) is a column that automatically generates numeric values. A start and increment value can be set, but most DBA leave these at 1. A GUID column also generates numbers; the value of this cannot be controlled. Identity/GUID columns do not need to be indexed.
What is DataWarehousing?
• Subject-oriented, meaning that the data in the database is organized so that all the data elements relating to the same real-world event or object are linked together;
• Time-variant, meaning that the changes to the data in the database are tracked and recorded so that reports can be produced showing changes over time;
• Non-volatile, meaning that data in the database is never over-written or deleted, once committed, the data is static, read-only, but retained for future reporting.
• Integrated, meaning that the database contains data from most or all of an organization’s operational applications, and that this data is made consistent.
SQL SERVER – 2008 – Interview Questions and Answers – Part 3
1) General Questions of SQL SERVER
2) Common Questions Asked
Which TCP/IP port does SQL Server run on? How can it be changed?
SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties -> Port number, both on client and the server.
What are the difference between clustered and a non-clustered index? (Read More Here)
A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.
A non clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.
What are the different index configurations a table can have?
A table can have one of the following index configurations:
• No indexes
• A clustered index
• A clustered index and many nonclustered indexes
• A nonclustered index
• Many nonclustered indexes
What are different types of Collation Sensitivity?
Case sensitivity – A and a, B and b, etc.
Accent sensitivity – a and á, o and ó, etc.
Kana Sensitivity – When Japanese kana characters Hiragana and Katakana are treated differently, it is called Kana sensitive.
Width sensitivity – A single-byte character (half-width) and the same character represented as a double-byte character (full-width) are treated differently than it is width sensitive. (Read More Here)
What is OLTP (Online Transaction Processing)?
In OLTP – online transaction processing systems relational database design use the discipline of data modeling and generally follow the Codd rules of data normalization in order to ensure absolute data integrity. Using these rules complex information is broken down into its most simple structures (a table) where all of the individual atomic level elements relate to each other and satisfy the normalization rules.
What’s the difference between a primary key and a unique key?
Both primary key and unique key enforces uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn’t allow NULLs, but unique key allows one NULL only. (Read More Here)
What is difference between DELETE & TRUNCATE commands?
Delete command removes the rows from a table based on the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table and there will be no data in the table after we run the truncate command.
TRUNCATE
• TRUNCATE is faster and uses fewer system and transaction log resources than DELETE.
• TRUNCATE removes the data by deallocating the data pages used to store the table’s data, and only the page deallocations are recorded in the transaction log.
• TRUNCATE removes all rows from a table, but the table structure, its columns, constraints, indexes and so on, remains. The counter used by an identity for new rows is reset to the seed for the column.
• You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint. Because TRUNCATE TABLE is not logged, it cannot activate a trigger.
• TRUNCATE cannot be rolled back.
• TRUNCATE is DDL Command.
• TRUNCATE Resets identity of the table
DELETE
• DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.
• If you want to retain the identity counter, use DELETE instead. If you want to remove table definition and its data, use the DROP TABLE statement.
• DELETE Can be used with or without a WHERE clause
• DELETE Activates Triggers.
• DELETE can be rolled back.
• DELETE is DML Command.
• DELETE does not reset identity of the table.
(Read More Here)
When is the use of UPDATE_STATISTICS command?
This command is basically used when a large processing of data has occurred. If a large amount of deletions any modification or Bulk Copy into the tables has occurred, it has to update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.
What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
They specify a search condition for a group or an aggregate. But the difference is that HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query whereas WHERE Clause is applied to each row before they are part of the GROUP BY function in a query. (Read More Here)
What are the properties and different Types of Sub-Queries?
Properties of Sub-Query
• A sub-query must be enclosed in the parenthesis.
• A sub-query must be put in the right hand of the comparison operator, and
• A sub-query cannot contain an ORDER-BY clause.
• A query can contain more than one sub-query.
Types of Sub-query
• Single-row sub-query, where the sub-query returns only one row.
• Multiple-row sub-query, where the sub-query returns multiple rows,. and
• Multiple column sub-query, where the sub-query returns multiple columns
What is SQL Profiler?
SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance of Microsoft SQL Server. You can capture and save data about each event to a file or SQL Server table to analyze later. For example, you can monitor a production environment to see which stored procedures are hampering performances by executing too slowly.
Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming too large, you can filter them based on the information you want, so that only a subset of the event data is collected. Monitoring too many events adds overhead to the server and the monitoring process and can cause the trace file or trace table to grow very large, especially when the monitoring process takes place over a long period of time.
What are the authentication modes in SQL Server? How can it be changed?
Windows mode and Mixed Mode – SQL & Windows.
To change authentication mode in SQL Server click Start, Programs, Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group. Select the server then from the Tools menu select SQL Server Configuration Properties, and choose the Security page.
SQL SERVER – 2008 – Interview Questions and Answers – Part 4
1) General Questions of SQL SERVER
Which command using Query Analyzer will give you the version of SQL server and operating system?
SELECT SERVERPROPERTY ('productversion'), SERVERPROPERTY ('productlevel'), SERVERPROPERTY ('edition')
What is SQL Server Agent?
SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the implementation of tasks for the DBA, with its full-function scheduling engine, which allows you to schedule your own jobs and scripts. (Read More Here)
Can a stored procedure call itself or recursive stored procedure? How much level SP nesting is possible?
Yes. Because Transact-SQL supports recursion, you can write stored procedures that call themselves. Recursion can be defined as a method of problem solving wherein the solution is arrived at by repetitively applying it to subsets of the problem. A common application of recursive logic is to perform numeric computations that lend themselves to repetitive evaluation by the same processing steps. Stored procedures are nested when one stored procedure calls another or executes managed code by referencing a CLR routine, type, or aggregate. You can nest stored procedures and managed code references up to 32 levels.
What is Log Shipping?
Log shipping is the process of automating the backup of database and transaction log files on a production SQL server, and then restoring them onto a standby server. Enterprise Editions only supports log shipping. In log shipping the transactional log file from one server is automatically updated into the backup database on the other server. If one server fails, the other server will have the same db and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs throughout the day and automatically restore them on the standby server at defined interval.
Name 3 ways to get an accurate count of the number of records in a table?
SELECT * FROM table1
SELECT COUNT(*) FROM table1
SELECT rows FROM sysindexes WHERE id = OBJECT_ID(table1) AND indid < 2
What does it mean to have QUOTED_IDENTIFIER ON? What are the implications of having it OFF?
When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation marks, and literals must be delimited by single quotation marks. When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all Transact-SQL rules for identifiers. (Read More Here)
What is the difference between a Local and a Global temporary table?
A local temporary table exists only for the duration of a connection or, if defined inside a compound statement, for the duration of the compound statement.
A global temporary table remains in the database permanently, but the rows exist only within a given connection. When connection is closed, the data in the global temporary table disappears. However, the table definition remains with the database for access when database is opened next time.
What is the STUFF function and how does it differ from the REPLACE function?
STUFF function is used to overwrite existing characters. Using this syntax, STUFF (string_expression, start, length, replacement_characters), string_expression is the string that will have characters substituted, start is the starting position, length is the number of characters in the string that are substituted, and replacement_characters are the new characters interjected into the string. REPLACE function to replace existing characters of all occurrences. Using the syntax REPLACE (string_expression, search_string, replacement_string), where every incidence of search_string found in the string_expression will be replaced with replacement_string.
What is PRIMARY KEY?
A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should have a primary key constraint to uniquely identify each row and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.
What is UNIQUE KEY constraint?
A UNIQUE constraint enforces the uniqueness of the values in a set of columns, so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.
What is FOREIGN KEY?
A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.
What is CHECK Constraint?
A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity. (Read More Here)
What is NOT NULL Constraint?
A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints.
(Read More Here)
How to get @@ERROR and @@ROWCOUNT at the same time?
If @@Rowcount is checked after Error checking statement then it will have 0 as the value of @@Recordcount as it would have been reset. And if @@Recordcount is checked before the error-checking statement then @@Error would get reset. To get @@error and @@rowcount at the same time do both in same statement and store them in local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR
What is a Scheduled Jobs or What is a Scheduled Tasks?
Scheduled tasks let user automate processes that run on regular or predictable cycles. User can schedule administrative tasks, such as cube processing, to run during times of slow business activity. User can also determine the order in which tasks run by creating job steps within a SQL Server Agent job. E.g. back up database, Update Stats of Tables. Job steps give user control over flow of execution. If one job fails, user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.
What are the advantages of using Stored Procedures?
• Stored procedure can reduced network traffic and latency, boosting application performance.
• Stored procedure execution plans can be reused, staying cached in SQL Server’s memory, reducing server overhead.
• Stored procedures help promote code reuse.
• Stored procedures can encapsulate logic. You can change stored procedure code without affecting clients.
• Stored procedures provide better security to your data.
What is a table called, if it has neither Cluster nor Non-cluster Index? What is it used for?
Unindexed table or Heap. Microsoft Press Books and Book on Line (BOL) refers it as Heap. A heap is a table that does not have a clustered index and, therefore, the pages are not linked by pointers. The IAM pages are the only structures that link the pages in a table together. Unindexed tables are good for fast storing of data. Many times it is better to drop all indexes from table and then do bulk of inserts and to restore those indexes after that.
Can SQL Servers linked to other servers like Oracle?
SQL Server can be linked to any server provided it has OLE-DB provider from Microsoft to allow a link. E.g. Oracle has an OLE-DB provider for oracle that Microsoft provides to add it as linked server to SQL Server group
What is BCP? When does it used?
BulkCopy is a tool used to copy huge amount of data from tables and views. BCP does not copy the structures same as source to destination. BULK INSERT command helps to import a data file into a database table or view in a user-specified format.
SQL SERVER – 2008 – Interview Questions and Answers – Part 5
What command do we use to rename a db, a table and a column?
To rename db
sp_renamedb 'oldname' , 'newname'
If someone is using db it will not accept sp_renmaedb. In that case first bring db to single user using sp_dboptions. Use sp_renamedb to rename database. Use sp_dboptions to bring database to multi user mode.
E.g.
USE master;
GO
EXEC sp_dboption AdventureWorks, 'Single User', True
GO
EXEC sp_renamedb 'AdventureWorks', 'AdventureWorks_New'
GO
EXEC sp_dboption AdventureWorks, 'Single User', False
GO
To rename Table
We can change the table name using sp_rename as follows,
sp_rename 'oldTableName' 'newTableName'
E.g.
sp_RENAME 'Table_First', 'Table_Last'
GO
To rename Column
The script for renaming any column :
sp_rename 'TableName.[OldcolumnName]', 'NewColumnName', 'Column'
E.g.
sp_RENAME 'Table_First.Name', 'NameChange' , 'COLUMN'
GO
What are sp_configure commands and set commands?
Use sp_configure to display or change server-level settings. To change database-level settings, use ALTER DATABASE. To change settings that affect only the current user session, use the SET statement.
E.g.
sp_CONFIGURE 'show advanced', 0
GO
RECONFIGURE
GO
sp_CONFIGURE
GO
You can run following command and check advance global configuration settings.
sp_CONFIGURE 'show advanced', 1
GO
RECONFIGURE
GO
sp_CONFIGURE
GO
(Read More Here)
How to implement one-to-one, one-to-many and many-to-many relationships while designing tables?
One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships.
Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table.
What is an execution plan? When would you use it? How would you view the execution plan?
An execution plan is basically a road map that graphically or textually shows the data retrieval methods chosen by the SQL Server query optimizer for a stored procedure or ad-hoc query and is a very useful tool for a developer to understand the performance characteristics of a query or stored procedure since the plan is the one that SQL Server will place in its cache and use to execute the stored procedure or query. From within Query Analyzer is an option called “Show Execution Plan” (located on the Query drop-down menu). If this option is turned on it will display query execution plan in separate window when query is ran again.
SQL SERVER – 2008 – Interview Questions and Answers – Part 7
How can we rewrite sub-queries into simple select statements or with joins?
Yes we can write using Common Table Expression (CTE). A Common Table Expression (CTE) is an expression that can be thought of as a temporary result set which is defined within the execution of a single SQL statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query.
E.g.
USE AdventureWorks
GO
WITH EmployeeDepartment_CTE AS (
SELECT EmployeeID,DepartmentID,ShiftID
FROM HumanResources.EmployeeDepartmentHistory
)
SELECT ecte.EmployeeId,ed.DepartmentID, ed.Name,ecte.ShiftID
FROM HumanResources.Department ed
INNER JOIN EmployeeDepartment_CTE ecte ON ecte.DepartmentID = ed.DepartmentID
GO
What is CLR?
In SQL Server 2008, SQL Server objects such as user-defined functions can be created using such CLR languages. This CLR language support extends not only to user-defined functions, but also to stored procedures and triggers. You can develop such CLR add-ons to SQL Server using Visual Studio 2008. (Read More Here)
What are synonyms?
Synonyms give you the ability to provide alternate names for database objects. You can alias object names; for example, using the Employee table as Emp. You can also shorten names. This is especially useful when dealing with three and four part names; for example, shortening server.database.owner.object to object. (Read More Here)
What is LINQ?
Language Integrated Query (LINQ) adds the ability to query objects using .NET languages. The LINQ to SQL object/relational mapping (O/RM) framework provides the following basic features:
• Tools to create classes (usually called entities) mapped to database tables
• Compatibility with LINQ’s standard query operations
• The DataContext class, with features such as entity record monitoring, automatic SQL statement generation, record concurrency detection, and much more
What is Isolation Levels?
Transactions specify an isolation level that defines the degree to which one transaction must be isolated from resource or data modifications made by other transactions. Isolation levels are described in terms of which concurrency side-effects, such as dirty reads or phantom reads, are allowed.
Transaction isolation levels control:
• Whether locks are taken when data is read, and what type of locks are requested.
• How long the read locks are held.
• Whether a read operation referencing rows modified by another transaction:
• Blocks until the exclusive lock on the row is freed.
• Retrieves the committed version of the row that existed at the time the statement or transaction started.
• Reads the uncommitted data modification. (Read More Here)
What is use of EXCEPT Clause?
EXCEPT clause is similar to MINUS operation in Oracle. The EXCEPT query and MINUS query returns all rows in the first query that are not returned in the second query. Each SQL statement within the EXCEPT query and MINUS query must have the same number of fields in the result sets with similar data types. (Read More Here)
What is XPath?
XPath uses a set of expressions to select nodes to be processed. The most common expression that you’ll use is the location path expression, which returns back a set of nodes called a node set. XPath can use both an unabbreviated and an abbreviated syntax. The following is the unabbreviated syntax for a location path:
/axisName::nodeTest[predicate]/axisName::nodeTest[predicate]
What is NOLOCK?
Using the NOLOCK query optimizer hint is generally considered good practice in order to improve concurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks are taken when data is read. The result is a Dirty Read, which means that another process could be updating the data at the exact time you are reading it. There are no guarantees that your query will retrieve the most recent data. The advantage to performance is that your reading of data will not block updates from taking place, and updates will not block your reading of data. SELECT statements take Shared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, but other processes are blocked from modifying the data. The updates will queue until all the reads have completed, and reads requested after the update will wait for the updates to complete. The result to your system is delay (blocking). (Read More Here)
How would you handle error in SQL SERVER 2008?
SQL Server now supports the use of TRY…CATCH constructs for providing rich error handling. TRY…CATCH lets us build error handling at the level we need, in the way we need to, by setting a region where if any error occurs, it will break out of the region and head to an error handler. The basic structure is as follows:
BEGIN TRY
END TRY
BEGIN CATCH
END CATCH
So if any error occurs in the TRY block, execution is diverted to the CATCH block, and the error can be dealt.
What is RAISEERROR?
RaiseError generates an error message and initiates error processing for the session. RAISERROR can either reference a user-defined message stored in the sys.messages catalog view or build a message dynamically. The message is returned as a server error message to the calling application or to an associated CATCH block of a TRY…CATCH construct. (Read More Here)
How to rebuild Master Databse?
Master database is system database and it contains information about running server’s configuration. When SQL Server 2005 is installed it usually creates master, model, msdb, tempdb resource and distribution system database by default. Only Master database is the one which is absolutely must have database. Without Master database SQL Server cannot be started. This is the reason it is extremely important to backup Master database.
To rebuild the Master database, Run Setup.exe, verify, and repair a SQL Server instance, and rebuild the system databases. This procedure is most often used to rebuild the master database for a corrupted installation of SQL Server.
What is XML Datatype?
The xml data type lets you store XML documents and fragments in a SQL Server database. An XML fragment is an XML instance that is missing a single top-level element. You can create columns and variables of the xml type and store XML instances in them. The xml data type and associated methods help integrate XML into the relational framework of SQL Server.
SQL SERVER – 2008 – Interview Questions and Answers – Part 8
What is Data Compression?
In SQL SERVE 2008 Data Compression comes in two flavors:
• Row Compression
• Page Compression
Row Compression
Row compression changes the format of physical storage of data. It minimize the metadata (column information, length, offsets etc) associated with each record. Numeric data types and fixed length strings are stored in variable-length storage format, just like Varchar. (Read More Here)
Page Compression
Page compression allows common data to be shared between rows for a given page. Its uses the following techniques to compress data:
• Row compression.
• Prefix Compression. For every column in a page duplicate prefixes are identified. These prefixes are saved in compression information headers (CI) which resides after page header. A reference number is assigned to these prefixes and that reference number is replaced where ever those prefixes are being used.
Dictionary Compression.
Dictionary compression searches for duplicate values throughout the page and stores them in CI. The main difference between prefix and dictionary compression is that prefix is only restricted to one column while dictionary is applicable to the complete page.
What is use of DBCC Commands?
The Transact-SQL programming language provides DBCC statements that act as Database Console Commands for SQL Server. DBCC commands are used to perform following tasks.
• Maintenance tasks on database, index, or filegroup.
• Tasks that gather and display various types of information.
• Validation operations on a database, table, index, catalog, filegroup, or allocation of database pages.
• Miscellaneous tasks such as enabling trace flags or removing a DLL from memory.
(Read More Here)
How to find tables without Indexes?
Run following query in Query Editor.
USE ;
GO
SELECT SCHEMA_NAME(schema_id) AS schema_name
,name AS table_name
FROM sys.tables
WHERE OBJECTPROPERTY(OBJECT_ID,'IsIndexed') = 0
ORDER BY schema_name, table_name;
GO
How to copy the tables, schema and views from one SQL Server to another?
There are multiple ways to do this.
1. “Detach Database” from one server and “Attach Database” to another server.
2. Manually script all the objects using SSMS and run the script on new server.
3. Use Wizard of SSMS. (Read More Here)
How to copy data from one table to another table?
There are multiple ways to do this.
1) INSERT INTO SELECT
This method is used when table is already created in the database earlier and data is to be inserted into this table from another table. If columns listed in insert clause and select clause are same, they are not required to list them.
2) SELECT INTO
This method is used when table is not created earlier and needs to be created when data from one table is to be inserted into newly created table from another table. New table is created with same data types as selected columns.
(Read More Here)
What is Catalog Views?
Catalog views return information that is used by the SQL Server Database Engine. Catalog Views are the most general interface to the catalog metadata and provide the most efficient way to obtain, transform, and present customized forms of this information. All user-available catalog metadata is exposed through catalog views.
What is PIVOT and UNPIVOT?
A Pivot Table can automatically sort, count, and total the data stored in one table or spreadsheet and create a second table displaying the summarized data. The PIVOT operator turns the values of a specified column into column names, effectively rotating a table.
UNPIVOT table is reverse of PIVOT Table. (Read More Here)
What is Filestream?
Filestream allows you to store large objects in the file system and have these files integrated within the database. It enables SQL Server based applications to store unstructured data such as documents, images, audios, videos etc. in the file system. FILESTREAM basically integrates the SQL Server Database Engine with New Technology File System (NTFS); it basically stores the data in varbinary (max) data type. Using this data type, the unstructured data is stored in the NTFS file system and the SQL Server Database Engine manages the link between the Filestream column and the actual file located in the NTFS. Using Transact SQL statements users can insert, update, delete and select the data stored in FILESTREAM enabled tables.
What is Dirty Read ?
A dirty read occurs when two operations say, read and write occurs together giving the incorrect or unedited data. Suppose, A has changed a row, but has not committed the changes. B reads the uncommitted data but his view of the data may be wrong so that is Dirty Read.
What is SQLCMD?
sqlcmd is enhanced version of the isql and osql and it provides way more functionality than other two options. In other words sqlcmd is better replacement of isql (which will be deprecated eventually) and osql (not included in SQL Server 2005 RTM). sqlcmd can work two modes – i) BATCH and ii) interactive modes. (Read More)
What is Aggregate Functions?
Aggregate functions perform a calculation on a set of values and return a single value. Aggregate functions ignore NULL values except COUNT function. HAVING clause is used, along with GROUP BY, for filtering query using aggregate values.
Following functions are aggregate functions.
AVG, MIN, CHECKSUM_AGG, SUM, COUNT, STDEV, COUNT_BIG, STDEVP, GROUPING, VAR, MAX, VARP (Read More Here )
What do you mean by Table Sample?
TABLESAMPLE allows you to extract a sampling of rows from a table in the FROM clause. The rows retrieved are random and they are not in any order. This sampling can be based on a percentage of number of rows. You can use TABLESAMPLE when only a sampling of rows is necessary for the application instead of a full result set. (Read More Here)
What is Row_Number()?
ROW_NUMBER() returns a column as an expression that contains the row’s number within the result set. This is only a number used in the context of the result set, if the result changes, the ROW_NUMBER() will change.
What are Ranking Functions?
Ranking functions return a ranking value for each row in a partition. All the ranking functions are non-deterministic. Different Ranking functions are:
ROW_NUMBER () OVER ([] )
Returns the sequential number of a row within a partition of a result set, starting at 1 for the first row in each partition.
RANK () OVER ([] )
Returns the rank of each row within the partition of a result set.
DENSE_RANK () OVER ([] )
Returns the rank of rows within the partition of a result set, without any gaps in the ranking. (Read More Here )
What is the difference between UNION and UNION ALL?
UNION
The UNION command is used to select related information from two tables, much like the JOIN command. However, when using the UNION command all selected columns need to be of the same data type. With UNION, only distinct values are selected.
UNION ALL
The UNION ALL command is equal to the UNION command, except that UNION ALL selects all values.
The difference between Union and Union all is that Union all will not eliminate duplicate rows, instead it just pulls all rows from all tables fitting your query specifics and combines them into a table. (Read More Here)
What is B-Tree?
The database server uses a B-tree structure to organize index information. B-Tree generally has following types of index pages or nodes:
• root node: A root node contains node pointers to branch nodes which can be only one.
• branch nodes: A branch node contains pointers to leaf nodes or other branch nodes which can be two or more.
• leaf nodes: A leaf node contains index items and horizontal pointers to other leaf nodes which can be many.
SQL SERVER – Outer Join Not Allowed in Indexed Views
I recently received an email that contains a question from one of my readers. I have already replied the answer to his email, but I would still like to bring it to your attention and ask if you think I could have done any better with the example I gave.
The question was raised when the email sender read the white paper, Improving Performance with SQL Server 2008 Indexed Views. If you scroll all the way down through the said white paper, there are several questions and answers.
Q: Why can’t I use OUTER JOIN in an Indexed view?
A: Rows can logically disappear from an Indexed view based on OUTER JOIN when you insert data into a base table. This makes the OUTER JOIN view to be increasingly updated, which is relatively difficult to implement. In addition, the performance of the implementation would be slower than for views based on standard (INNER) JOIN.
The reader was confused with my answer and wanted me to explain it further. Here is the example which I have quickly put together to demonstrate the behavior described in the above statement.
USE tempdb
GO
-- Create Two Tables
CREATE TABLE BaseTable (ID1 INT, Col1 VARCHAR(100))
CREATE TABLE JoinedTable (ID2 INT, Col2 VARCHAR(100))
GO
-- Insert Values in Tables
INSERT INTO BaseTable (ID1,Col1)
SELECT 1,'First'
UNION ALL
SELECT 2,'Second'
GO
INSERT INTO JoinedTable (ID2,Col2)
SELECT 1,'First'
UNION ALL
SELECT 2,'Second'
UNION ALL
SELECT 3,'Third'
UNION ALL
SELECT 4,'Fourth'
GO
-- Use Outer Join
SELECT jt.*
FROM BaseTable bt
RIGHT OUTER JOIN JoinedTable jt ON bt.ID1 = jt.ID2
WHERE bt.ID1 IS NULL
GO
The script above will give us the following output:
-- Now Insert Rows in Base Table
INSERT INTO BaseTable (ID1,Col1)
SELECT 3,'Third'
GO
-- You will notice that one row less retrieved from Join
SELECT jt.*
FROM BaseTable bt
RIGHT OUTER JOIN JoinedTable jt ON bt.ID1 = jt.ID2
WHERE bt.ID1 IS NULL
GO
-- Clean up
DROP TABLE BaseTable
DROP TABLE JoinedTable
GO
After running this script, you will notice that as the base table gains one row, the result loses one row. Going back to the white paper mentioned earlier, I believe this is expensive to manage for the same reason why it is not allowed in Indexed View.
The question was raised when the email sender read the white paper, Improving Performance with SQL Server 2008 Indexed Views. If you scroll all the way down through the said white paper, there are several questions and answers.
Q: Why can’t I use OUTER JOIN in an Indexed view?
A: Rows can logically disappear from an Indexed view based on OUTER JOIN when you insert data into a base table. This makes the OUTER JOIN view to be increasingly updated, which is relatively difficult to implement. In addition, the performance of the implementation would be slower than for views based on standard (INNER) JOIN.
The reader was confused with my answer and wanted me to explain it further. Here is the example which I have quickly put together to demonstrate the behavior described in the above statement.
USE tempdb
GO
-- Create Two Tables
CREATE TABLE BaseTable (ID1 INT, Col1 VARCHAR(100))
CREATE TABLE JoinedTable (ID2 INT, Col2 VARCHAR(100))
GO
-- Insert Values in Tables
INSERT INTO BaseTable (ID1,Col1)
SELECT 1,'First'
UNION ALL
SELECT 2,'Second'
GO
INSERT INTO JoinedTable (ID2,Col2)
SELECT 1,'First'
UNION ALL
SELECT 2,'Second'
UNION ALL
SELECT 3,'Third'
UNION ALL
SELECT 4,'Fourth'
GO
-- Use Outer Join
SELECT jt.*
FROM BaseTable bt
RIGHT OUTER JOIN JoinedTable jt ON bt.ID1 = jt.ID2
WHERE bt.ID1 IS NULL
GO
The script above will give us the following output:
-- Now Insert Rows in Base Table
INSERT INTO BaseTable (ID1,Col1)
SELECT 3,'Third'
GO
-- You will notice that one row less retrieved from Join
SELECT jt.*
FROM BaseTable bt
RIGHT OUTER JOIN JoinedTable jt ON bt.ID1 = jt.ID2
WHERE bt.ID1 IS NULL
GO
-- Clean up
DROP TABLE BaseTable
DROP TABLE JoinedTable
GO
After running this script, you will notice that as the base table gains one row, the result loses one row. Going back to the white paper mentioned earlier, I believe this is expensive to manage for the same reason why it is not allowed in Indexed View.
SQL SERVER – The Self Join – Inner Join and Outer Join
Self Join has always been an note-worthy case. It is interesting to ask questions on self join in a room full of developers. I often ask – if there are three kind of joins, i.e.- Inner Join, Outer Join and Cross Join; what type of join is Self Join? The usual answer is that it is an Inner Join. In fact, it can be classified under any type of join. I have previously written about this in my interview questions and answers series. I have also mentioned this subject when I explained the joins in detail over SQL SERVER – Introduction to JOINs – Basic of JOINs.
When I mention that Self Join can be the outer join, I often get a request for an example for the same. I have created example using AdventureWorks Database of Self Join earlier, but that was meant for inner join as well. Let us create a new example today, where we will see how Self Join can be implemented as an Inner Join as well as Outer Join.
Let us first create the same table for an employee. One of the columns in the same table contains the ID of manger, who is also an employee for the same company. This way, all the employees and their managers are present in the same table. If we want to find the manager of a particular employee, we need use self join.
USE TempDb
GO
-- Create a Table
CREATE TABLE Employee(
EmployeeID INT PRIMARY KEY,
Name NVARCHAR(50),
ManagerID INT
)
GO
http://www.pinaldave.com/bimg/selfjoini1.jpg
-- Insert Sample Data
INSERT INTO Employee
SELECT 1, 'Mike', 3
UNION ALL
SELECT 2, 'David', 3
UNION ALL
SELECT 3, 'Roger', NULL
UNION ALL
SELECT 4, 'Marry',2
UNION ALL
SELECT 5, 'Joseph',2
UNION ALL
SELECT 7, 'Ben',2
GO
-- Check the data
SELECT *
FROM Employee
GO
We will now use inner join to find the employees and their managers’ details.
-- Inner Join
SELECT e1.Name EmployeeName, e2.name AS ManagerName
FROM Employee e1
INNER JOIN Employee e2
ON e1.ManagerID = e2.EmployeeID
GO
From the result set, we can see that all the employees who have a manager are visible. However we are unable to find out the top manager of the company as he is not visible in our resultset. The reason for the same is that due to inner join, his name is filtered out. Inner join does not bring any result which does not have manager id. Let us convert Inner Join to Outer Join and then see the resultset.
-- Outer Join
SELECT e1.Name EmployeeName, ISNULL(e2.name, 'Top Manager') AS ManagerName
FROM Employee e1
LEFT JOIN Employee e2
ON e1.ManagerID = e2.EmployeeID
GO
Once we convert Inner Join to Outer Join, we can see the Top Manager as well. Here we have seen how Self Join can behave as an inner join as well as an outer join.
When I mention that Self Join can be the outer join, I often get a request for an example for the same. I have created example using AdventureWorks Database of Self Join earlier, but that was meant for inner join as well. Let us create a new example today, where we will see how Self Join can be implemented as an Inner Join as well as Outer Join.
Let us first create the same table for an employee. One of the columns in the same table contains the ID of manger, who is also an employee for the same company. This way, all the employees and their managers are present in the same table. If we want to find the manager of a particular employee, we need use self join.
USE TempDb
GO
-- Create a Table
CREATE TABLE Employee(
EmployeeID INT PRIMARY KEY,
Name NVARCHAR(50),
ManagerID INT
)
GO
http://www.pinaldave.com/bimg/selfjoini1.jpg
-- Insert Sample Data
INSERT INTO Employee
SELECT 1, 'Mike', 3
UNION ALL
SELECT 2, 'David', 3
UNION ALL
SELECT 3, 'Roger', NULL
UNION ALL
SELECT 4, 'Marry',2
UNION ALL
SELECT 5, 'Joseph',2
UNION ALL
SELECT 7, 'Ben',2
GO
-- Check the data
SELECT *
FROM Employee
GO
We will now use inner join to find the employees and their managers’ details.
-- Inner Join
SELECT e1.Name EmployeeName, e2.name AS ManagerName
FROM Employee e1
INNER JOIN Employee e2
ON e1.ManagerID = e2.EmployeeID
GO
From the result set, we can see that all the employees who have a manager are visible. However we are unable to find out the top manager of the company as he is not visible in our resultset. The reason for the same is that due to inner join, his name is filtered out. Inner join does not bring any result which does not have manager id. Let us convert Inner Join to Outer Join and then see the resultset.
-- Outer Join
SELECT e1.Name EmployeeName, ISNULL(e2.name, 'Top Manager') AS ManagerName
FROM Employee e1
LEFT JOIN Employee e2
ON e1.ManagerID = e2.EmployeeID
GO
Once we convert Inner Join to Outer Join, we can see the Top Manager as well. Here we have seen how Self Join can behave as an inner join as well as an outer join.
Overview of Error Handling in SQL Server 2005
Table of Contents
* Introduction
* When We Need To Handle Error in SQL Server
* Error Handling Mechanism
* Using @@ERROR
o General Syntax
o Return Type
o Sample Example
o When We Should Use @@Error
* Using Try...Catch Block
o General Syntax
o Sample Example
o Nested TRY-CATCH Block
o Try-Catch Block for Transaction Roll Back
* Future Study
* History
Introduction
During development of any application, one of the most common things we need to take care of is Exception and Error handling. Similarly we need to take care of handling error and exception while designing our database like inside stored procedure. In SQL Server 2005, there are some beautiful features available using which we can handle the error.
When We Need To Handle Error in SQL Server
Generally a developer tries to handle all kinds of exception from the code itself. But sometimes we need to handle the same from the DB site itself. There are some scenarios like, we are expecting some rows should come when we will execute the store procedure, but unfortunately SP returns none of them. Below points can be some possible scenarios where we can use error handling:
* While executing some DML Statement like INSERT, DELETE, UPDATE we can handle the error for checking proper output
* If transaction fails, then we need to rollback - This can be done by error handling
* While using Cursor in SQL Server
Error Handling Mechanism
The two most common mechanisms for error handling in SQL Server 2005 are:
* @@ERROR
* TRY-CATCH Block
Let's have a look at how we can implement both @@Error and Try-Catch block to handle the error in SQL Server 2005.
Using @@ERROR
We can consider @@ERROR as one of the basic error handling mechanisms in SQL Server. @@Error is a Global Variable in SQL Server. This variable automatically populates the error message when a certain error occurred in any statement. But we have to trace it within just after the next line where the actual error occurred, otherwise, it will reset to 0.
General Syntax
General syntax for @@ERROR is as follows:
Collapse
Select @@ERROR
Return Type
Collapse
int
It returns the Error Number.
Sample Example
I have a table named StudentDetails with columns, Roll (int), Name (varchar) and Address (varchar). Now I am intentionally trying to insert a char in Roll field:
Collapse
insert into StudentDetails (roll,[Name],Address)
values ('a','Abhijit','India')
This will throw the following Error :
Collapse
Msg 245, Level 16, State 1, Line 1
Conversion failed when converting the varchar value 'a' to data type int.
Check out the Message and number, it is 245. Now, I am executing the @@Error statement just after this statement and check out the output:
Collapse
Select @@Error
The output is:
Error1.JPG
So, @@Error returns the same error as return by insert command. As I have already said, @@Error returns the error number for the last Transact-SQL statement executed, so if we execute any @@Error statement, we will get output 0.
When We Should Use @@Error
There are some scenarios where we should use @@ERROR:
* With Insert, Delete, Update, Select Into Statement
* While using Cursor in SQL Server (Open, Fetch Cursor)
* While executing any Stored Procedure
Using Try...Catch Block
This is available from SQL Server 2005 Onwards. This is generally used where want to trap or catch error for multiple SQL statements like or a SQL Block of statement. TRY...CATCH blocks are the standard approach to exception handling in modern programming languages. Use and syntax are likely the same as normal programming language. Like Exception Handling in Programming Language, we can use nested Try-Catch block in SQL Server also.
Try block will catch the error and will throw it in the Catch block. Catch block then handles the scenario.
GeneralBlock.JPG
General Syntax
Below is the general syntax for Try-Catch block:
Collapse
-- SQL Statement
-- SQL Statement
BEGIN TRY
-- SQL Statement or Block
END TRY
BEGIN CATCH
-- SQL Statement or Block
END CATCH
-- SQL Statement
Whenever there are some errors in TRY Block, execution will moved to CATCH block.
Sample Example
As I have already discussed about the studentDetails table, I am now going to insert one record in the table with Roll='a'.
Collapse
BEGIN TRY
INSERT INTO StudentDetails(Roll, [Name])
VALUES('a', 'Abhijit')
END TRY
BEGIN CATCH
SELECT 'There was an error while Inserting records in DB '
END CATCH
As Roll is an int type but I am trying to insert a char type data which will violate the type conversion rule, an error will be thrown. So the execution pointer will jump to Catch block. And below is the output:
Collapse
There was an error while Inserting records in DB
Now, to get the details of the error SQL Server provides the following System function that we can use inside our Catch-block for retrieving the details of the error. Please check the below table:
Function Name Description
ERROR_MESSAGE() Returns the complete description of the error message
ERROR_NUMBER() Returns the number of the error
ERROR_SEVERITY() Returns the number of the Severity
ERROR_STATE() Returns the error state number
ERROR_PROCEDURE() Returns the name of the stored procedure where the error occurred
ERROR_LINE() Returns the line number that caused the error
Here is one simple example of using System Function:
Collapse
BEGIN TRY
INSERT INTO StudentDetails(Roll, [Name])
VALUES('a', 'Abhijit')
END TRY
BEGIN CATCH
SELECT ' Error Message: ' + ERROR_MESSAGE() as ErrorDescription
END CATCH
I have executed the same code block here but rather than showing custom message, I am showing the internal Error message by Calling ERROR_MESSAGE() System function. Below is the output:
SystemFunction.JPG
Nested TRY-CATCH Block
Like other programming languages, we can use Nested Try catch block in SQL Server 2005.
Collapse
BEGIN TRY
print 'At Outer Try Block'
BEGIN TRY
print 'At Inner Try Block'
END TRY
BEGIN CATCH
print 'At Inner catch Block'
END CATCH
END TRY
BEGIN CATCH
print 'At Outer catch block'
END CATCH
If we execute this, the output will look like:
Collapse
At Outer Try Block
At Inner Try Block
Now, Inner catch blocks throw an error:
Collapse
BEGIN TRY
print 'At Outer Try Block'
BEGIN TRY
print 'At Inner Try Block'
INSERT INTO StudentDetails(Roll, [Name]) _
VALUES('a', 'Abhijit') -- Throwing Exception
END TRY
BEGIN CATCH
print 'At Inner catch Block'
END CATCH
END TRY
BEGIN CATCH
print 'At Outer catch block'
END CATCH
Which gives the following output:
Collapse
At Outer Try Block
At Inner Try Block
At Inner catch Block
Try-Catch Block For Transaction Roll Back
Here I am going to explain one real life scenario of using TRY-CATCH block. One of the common scenarios is using Transaction. In a Transaction, we can have multiple operations. If all operations executed successfully, then database will commit otherwise we need to ROLLBACK.
Collapse
/*
I want to delete a Particular Records from Both Student
Details and Library. Database will only commit, iff both
delete statement execute successfully, If fails it will Roll
back. Intentionally I have passed a wrong roll ( Which causes)
the exception and transaction will rollback.
*/
BEGIN TRY
-- Start A Transaction
BEGIN TRANSACTION
-- Delete Student From StudenDetails Table
DELETE FROM StudentDetails WHERE Roll = '1'
Print 'Delete Record from Student Details Table'
-- Delete The Same Student Records From Library Table also
DELETE FROM Library WHERE Roll = 'a'
Print 'Delete Record from Library Table'
-- Commit if Both Success
COMMIT
-- Update Log Details
Insert into LogDetails(ID,Details) values ('1','Transaction Successful');
END TRY
BEGIN CATCH
Print 'Transaction Failed - Will Rollback'
-- Any Error Occurred during Transaction. Rollback
IF @@TRANCOUNT > 0
ROLLBACK -- Roll back
END CATCH
Below is the output:
Collapse
Delete Record from Student Details Table
Transaction Failed - Will Rollback
* Introduction
* When We Need To Handle Error in SQL Server
* Error Handling Mechanism
* Using @@ERROR
o General Syntax
o Return Type
o Sample Example
o When We Should Use @@Error
* Using Try...Catch Block
o General Syntax
o Sample Example
o Nested TRY-CATCH Block
o Try-Catch Block for Transaction Roll Back
* Future Study
* History
Introduction
During development of any application, one of the most common things we need to take care of is Exception and Error handling. Similarly we need to take care of handling error and exception while designing our database like inside stored procedure. In SQL Server 2005, there are some beautiful features available using which we can handle the error.
When We Need To Handle Error in SQL Server
Generally a developer tries to handle all kinds of exception from the code itself. But sometimes we need to handle the same from the DB site itself. There are some scenarios like, we are expecting some rows should come when we will execute the store procedure, but unfortunately SP returns none of them. Below points can be some possible scenarios where we can use error handling:
* While executing some DML Statement like INSERT, DELETE, UPDATE we can handle the error for checking proper output
* If transaction fails, then we need to rollback - This can be done by error handling
* While using Cursor in SQL Server
Error Handling Mechanism
The two most common mechanisms for error handling in SQL Server 2005 are:
* @@ERROR
* TRY-CATCH Block
Let's have a look at how we can implement both @@Error and Try-Catch block to handle the error in SQL Server 2005.
Using @@ERROR
We can consider @@ERROR as one of the basic error handling mechanisms in SQL Server. @@Error is a Global Variable in SQL Server. This variable automatically populates the error message when a certain error occurred in any statement. But we have to trace it within just after the next line where the actual error occurred, otherwise, it will reset to 0.
General Syntax
General syntax for @@ERROR is as follows:
Collapse
Select @@ERROR
Return Type
Collapse
int
It returns the Error Number.
Sample Example
I have a table named StudentDetails with columns, Roll (int), Name (varchar) and Address (varchar). Now I am intentionally trying to insert a char in Roll field:
Collapse
insert into StudentDetails (roll,[Name],Address)
values ('a','Abhijit','India')
This will throw the following Error :
Collapse
Msg 245, Level 16, State 1, Line 1
Conversion failed when converting the varchar value 'a' to data type int.
Check out the Message and number, it is 245. Now, I am executing the @@Error statement just after this statement and check out the output:
Collapse
Select @@Error
The output is:
Error1.JPG
So, @@Error returns the same error as return by insert command. As I have already said, @@Error returns the error number for the last Transact-SQL statement executed, so if we execute any @@Error statement, we will get output 0.
When We Should Use @@Error
There are some scenarios where we should use @@ERROR:
* With Insert, Delete, Update, Select Into Statement
* While using Cursor in SQL Server (Open, Fetch Cursor)
* While executing any Stored Procedure
Using Try...Catch Block
This is available from SQL Server 2005 Onwards. This is generally used where want to trap or catch error for multiple SQL statements like or a SQL Block of statement. TRY...CATCH blocks are the standard approach to exception handling in modern programming languages. Use and syntax are likely the same as normal programming language. Like Exception Handling in Programming Language, we can use nested Try-Catch block in SQL Server also.
Try block will catch the error and will throw it in the Catch block. Catch block then handles the scenario.
GeneralBlock.JPG
General Syntax
Below is the general syntax for Try-Catch block:
Collapse
-- SQL Statement
-- SQL Statement
BEGIN TRY
-- SQL Statement or Block
END TRY
BEGIN CATCH
-- SQL Statement or Block
END CATCH
-- SQL Statement
Whenever there are some errors in TRY Block, execution will moved to CATCH block.
Sample Example
As I have already discussed about the studentDetails table, I am now going to insert one record in the table with Roll='a'.
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BEGIN TRY
INSERT INTO StudentDetails(Roll, [Name])
VALUES('a', 'Abhijit')
END TRY
BEGIN CATCH
SELECT 'There was an error while Inserting records in DB '
END CATCH
As Roll is an int type but I am trying to insert a char type data which will violate the type conversion rule, an error will be thrown. So the execution pointer will jump to Catch block. And below is the output:
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There was an error while Inserting records in DB
Now, to get the details of the error SQL Server provides the following System function that we can use inside our Catch-block for retrieving the details of the error. Please check the below table:
Function Name Description
ERROR_MESSAGE() Returns the complete description of the error message
ERROR_NUMBER() Returns the number of the error
ERROR_SEVERITY() Returns the number of the Severity
ERROR_STATE() Returns the error state number
ERROR_PROCEDURE() Returns the name of the stored procedure where the error occurred
ERROR_LINE() Returns the line number that caused the error
Here is one simple example of using System Function:
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BEGIN TRY
INSERT INTO StudentDetails(Roll, [Name])
VALUES('a', 'Abhijit')
END TRY
BEGIN CATCH
SELECT ' Error Message: ' + ERROR_MESSAGE() as ErrorDescription
END CATCH
I have executed the same code block here but rather than showing custom message, I am showing the internal Error message by Calling ERROR_MESSAGE() System function. Below is the output:
SystemFunction.JPG
Nested TRY-CATCH Block
Like other programming languages, we can use Nested Try catch block in SQL Server 2005.
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BEGIN TRY
print 'At Outer Try Block'
BEGIN TRY
print 'At Inner Try Block'
END TRY
BEGIN CATCH
print 'At Inner catch Block'
END CATCH
END TRY
BEGIN CATCH
print 'At Outer catch block'
END CATCH
If we execute this, the output will look like:
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At Outer Try Block
At Inner Try Block
Now, Inner catch blocks throw an error:
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BEGIN TRY
print 'At Outer Try Block'
BEGIN TRY
print 'At Inner Try Block'
INSERT INTO StudentDetails(Roll, [Name]) _
VALUES('a', 'Abhijit') -- Throwing Exception
END TRY
BEGIN CATCH
print 'At Inner catch Block'
END CATCH
END TRY
BEGIN CATCH
print 'At Outer catch block'
END CATCH
Which gives the following output:
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At Outer Try Block
At Inner Try Block
At Inner catch Block
Try-Catch Block For Transaction Roll Back
Here I am going to explain one real life scenario of using TRY-CATCH block. One of the common scenarios is using Transaction. In a Transaction, we can have multiple operations. If all operations executed successfully, then database will commit otherwise we need to ROLLBACK.
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/*
I want to delete a Particular Records from Both Student
Details and Library. Database will only commit, iff both
delete statement execute successfully, If fails it will Roll
back. Intentionally I have passed a wrong roll ( Which causes)
the exception and transaction will rollback.
*/
BEGIN TRY
-- Start A Transaction
BEGIN TRANSACTION
-- Delete Student From StudenDetails Table
DELETE FROM StudentDetails WHERE Roll = '1'
Print 'Delete Record from Student Details Table'
-- Delete The Same Student Records From Library Table also
DELETE FROM Library WHERE Roll = 'a'
Print 'Delete Record from Library Table'
-- Commit if Both Success
COMMIT
-- Update Log Details
Insert into LogDetails(ID,Details) values ('1','Transaction Successful');
END TRY
BEGIN CATCH
Print 'Transaction Failed - Will Rollback'
-- Any Error Occurred during Transaction. Rollback
IF @@TRANCOUNT > 0
ROLLBACK -- Roll back
END CATCH
Below is the output:
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Delete Record from Student Details Table
Transaction Failed - Will Rollback
SQL SERVER – @@IDENTITY vs SCOPE_IDENTITY() vs IDENT_CURRENT – Retrieve Last Inserted Identity of Record
SELECT @@IDENTITY
It returns the last IDENTITY value produced on a connection, regardless of the table that produced the value, and regardless of the scope of the statement that produced the value.
@@IDENTITY will return the last identity value entered into a table in your current session. While @@IDENTITY is limited to the current session, it is not limited to the current scope. If you have a trigger on a table that causes an identity to be created in another table, you will get the identity that was created last, even if it was the trigger that created it.
SELECT SCOPE_IDENTITY()
It returns the last IDENTITY value produced on a connection and by a statement in the same scope, regardless of the table that produced the value.
SCOPE_IDENTITY(), like @@IDENTITY, will return the last identity value created in the current session, but it will also limit it to your current scope as well. In other words, it will return the last identity value that you explicitly created, rather than any identity that was created by a trigger or a user defined function.
SELECT IDENT_CURRENT(‘tablename’)
It returns the last IDENTITY value produced in a table, regardless of the connection that created the value, and regardless of the scope of the statement that produced the value.
IDENT_CURRENT is not limited by scope and session; it is limited to a specified table. IDENT_CURRENT returns the identity value generated for a specific table in any session and any scope.
To avoid the potential problems associated with adding a trigger later on, always use SCOPE_IDENTITY() to return the identity of the recently added row in your T SQL Statement or Stored Procedure.
It returns the last IDENTITY value produced on a connection, regardless of the table that produced the value, and regardless of the scope of the statement that produced the value.
@@IDENTITY will return the last identity value entered into a table in your current session. While @@IDENTITY is limited to the current session, it is not limited to the current scope. If you have a trigger on a table that causes an identity to be created in another table, you will get the identity that was created last, even if it was the trigger that created it.
SELECT SCOPE_IDENTITY()
It returns the last IDENTITY value produced on a connection and by a statement in the same scope, regardless of the table that produced the value.
SCOPE_IDENTITY(), like @@IDENTITY, will return the last identity value created in the current session, but it will also limit it to your current scope as well. In other words, it will return the last identity value that you explicitly created, rather than any identity that was created by a trigger or a user defined function.
SELECT IDENT_CURRENT(‘tablename’)
It returns the last IDENTITY value produced in a table, regardless of the connection that created the value, and regardless of the scope of the statement that produced the value.
IDENT_CURRENT is not limited by scope and session; it is limited to a specified table. IDENT_CURRENT returns the identity value generated for a specific table in any session and any scope.
To avoid the potential problems associated with adding a trigger later on, always use SCOPE_IDENTITY() to return the identity of the recently added row in your T SQL Statement or Stored Procedure.
SQL SERVER – 2005 SQL SERVER – 2005 – Explanation of TRY…CATCH and ERROR Handling With RAISEERROR Function
One of the developer at my company thought that we can not use RAISEERROR function in new feature of SQL Server 2005 TRY…CATCH. When asked for explanation he suggested SQL SERVER – 2005 Explanation of TRY…CATCH and ERROR Handling article as excuse suggesting that I did not give example of RAISEERROR with TRY…CATCH. We all thought it was funny. Just to keep record straight, TRY…CATCH can sure use RAISEERROR function.
First read original article for additional information about how TRY…CATCH works with ERROR codes. SQL SERVER – 2005 Explanation of TRY…CATCH and ERROR Handling
Example 1 : Simple TRY…CATCH without RAISEERROR function
BEGIN TRY
DECLARE @MyInt INT;
-- Following statement will create Devide by Zero Error
SET @MyInt = 1/0;
END TRY
BEGIN CATCH
SELECT 'Divide by zero error encountered.' ErrorMessage
END CATCH;
GO
ResultSet:
ErrorMessage
———————————
Divide by zero error encountered.
Example 2 : Simple TRY…CATCH with RAISEERROR function
BEGIN TRY
DECLARE @MyInt INT;
-- Following statement will create Devide by Zero Error
SET @MyInt = 1/0;
END TRY
BEGIN CATCH
DECLARE @ErrorMessage NVARCHAR(4000);
SELECT @ErrorMessage = ERROR_MESSAGE();
RAISERROR (@ErrorMessage, 16, 1);
END CATCH;
GO
ResultSet:
Msg 50000, Level 16, State 1, Line 9
Divide by zero error encountered.
First read original article for additional information about how TRY…CATCH works with ERROR codes. SQL SERVER – 2005 Explanation of TRY…CATCH and ERROR Handling
Example 1 : Simple TRY…CATCH without RAISEERROR function
BEGIN TRY
DECLARE @MyInt INT;
-- Following statement will create Devide by Zero Error
SET @MyInt = 1/0;
END TRY
BEGIN CATCH
SELECT 'Divide by zero error encountered.' ErrorMessage
END CATCH;
GO
ResultSet:
ErrorMessage
———————————
Divide by zero error encountered.
Example 2 : Simple TRY…CATCH with RAISEERROR function
BEGIN TRY
DECLARE @MyInt INT;
-- Following statement will create Devide by Zero Error
SET @MyInt = 1/0;
END TRY
BEGIN CATCH
DECLARE @ErrorMessage NVARCHAR(4000);
SELECT @ErrorMessage = ERROR_MESSAGE();
RAISERROR (@ErrorMessage, 16, 1);
END CATCH;
GO
ResultSet:
Msg 50000, Level 16, State 1, Line 9
Divide by zero error encountered.
SQL SERVER – 2005 Comparison SP_EXECUTESQL vs EXECUTE/EXEC
SQL SERVER – 2005 Comparison SP_EXECUTESQL vs EXECUTE/EXEC
Common Properties of SP_EXECUTESQL and EXECUTE/EXEC
* The Transact-SQL statements in the sp_executesql or EXECUTE string are not compiled into an execution plan until sp_executesql or the EXECUTE statement are executed. The strings are not parsed or checked for errors until they are executed. The names referenced in the strings are not resolved until they are executed.
* The Transact-SQL statements in the executed string do not have access to any of the variables declared in the batch that contains thesp_executesql or EXECUTE statement. The batch containing the sp_executesql or EXECUTE statement does not have access to variables or local cursors defined in the executed string.
* If the executed string has a USE statement that changes the database context, the change to the database context only lasts untilsp_executesql or the EXECUTE statement completes.
Comparison SP_EXECUTESQL vs EXECUTE/EXEC
sp_executesql gives you the possibility to use parameterised statements, EXECUTE does not. Parameterised statements gives no risk to SQL injection and also gives advantage of cached query plan. The sp_executesql stored procedure supports parameters. So, using the sp_executesql stored procedure instead of the EXECUTE statement improve readability of your code when there are many parameters are used. When you use thesp_executesql stored procedure to executes a Transact-SQL statements that will be reused many times, the SQL Server query optimizer will reuse the execution plan it generates for the first execution when the change in parameter values to the statement is the only variation.
sp_executesql can be used instead of stored procedures to execute a Transact-SQL statement a number of times when the change in parameter values to the statement is the only variation. Because the Transact-SQL statement itself remains constant and only the parameter values change, the SQL Server query optimizer is likely to reuse the execution plan it generates for the first execution.
Use SP_EXECUTESQL rather than EXEC(), it has better performance and improved security.
The syntax for sp_executesql for SQL Server 2005 is
sp_executesql [ @stmt = ] stmt
[
{, [@params=] N'@parameter_name data_type [ OUT | OUTPUT ][,...n]' }
{, [ @param1 = ] 'value1' [ ,...n ] }
]
The size of the string is limited only by available database server memory. On 64-bit servers, the size of the string is limited to 2 GB, the maximum size of nvarchar(max).
Common Properties of SP_EXECUTESQL and EXECUTE/EXEC
* The Transact-SQL statements in the sp_executesql or EXECUTE string are not compiled into an execution plan until sp_executesql or the EXECUTE statement are executed. The strings are not parsed or checked for errors until they are executed. The names referenced in the strings are not resolved until they are executed.
* The Transact-SQL statements in the executed string do not have access to any of the variables declared in the batch that contains thesp_executesql or EXECUTE statement. The batch containing the sp_executesql or EXECUTE statement does not have access to variables or local cursors defined in the executed string.
* If the executed string has a USE statement that changes the database context, the change to the database context only lasts untilsp_executesql or the EXECUTE statement completes.
Comparison SP_EXECUTESQL vs EXECUTE/EXEC
sp_executesql gives you the possibility to use parameterised statements, EXECUTE does not. Parameterised statements gives no risk to SQL injection and also gives advantage of cached query plan. The sp_executesql stored procedure supports parameters. So, using the sp_executesql stored procedure instead of the EXECUTE statement improve readability of your code when there are many parameters are used. When you use thesp_executesql stored procedure to executes a Transact-SQL statements that will be reused many times, the SQL Server query optimizer will reuse the execution plan it generates for the first execution when the change in parameter values to the statement is the only variation.
sp_executesql can be used instead of stored procedures to execute a Transact-SQL statement a number of times when the change in parameter values to the statement is the only variation. Because the Transact-SQL statement itself remains constant and only the parameter values change, the SQL Server query optimizer is likely to reuse the execution plan it generates for the first execution.
Use SP_EXECUTESQL rather than EXEC(), it has better performance and improved security.
The syntax for sp_executesql for SQL Server 2005 is
sp_executesql [ @stmt = ] stmt
[
{, [@params=] N'@parameter_name data_type [ OUT | OUTPUT ][,...n]' }
{, [ @param1 = ] 'value1' [ ,...n ] }
]
The size of the string is limited only by available database server memory. On 64-bit servers, the size of the string is limited to 2 GB, the maximum size of nvarchar(max).
Monday, April 12, 2010
How to close a form in asp.net
on page_load write add this code:
btnClose.Attributes.Add("onclick", "window.close();");
on Button_Click add this code:
Response.Write("");
btnClose.Attributes.Add("onclick", "window.close();");
on Button_Click add this code:
Response.Write("");
Tuesday, March 16, 2010
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