SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers) [Page – 5]

15 Friday May 2015

Posted by Pawan Kumar Khowal in Download SQL Interview Q's, SQL Server Interview Questions

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SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers)

Download – Pawan Kumar Khowal – SQL SERVER Interview Questions with Answers

PAGE – 5

Question 41. What is the best value for MAXDOP value?

Answer-

MAXDOP is Max degree of parallelism. This option controls the number of processors that are used for the execution of a query in a parallel plan.

One can use the max degree of parallelism option to limit the number of processors to use for parallel plan execution and to prevent run-away queries from impacting SQL Server performance by using all available CPUs.

The answer is: It depends. It depends on the hardware, the environment (OLTP vs. OLAP), and the load and so on.

The default value for MAXDOP is 0 (zero) and can be set or viewed using (sp_configure). A value of 0 means that SQL Server will use all processors if a query runs in parallel.

For details please visit –
http://www.mssqltips.com/sqlservertip/2650/what-maxdop-setting-should-be-used-for-sql-server/

Question 42. Which is better “Left Outer” Or “NOT EXIST”?

Answer-

In SQL Server, NOT EXISTS and NOT IN predicates are the best ways to search for missing values, as long as both columns in question are NOT NULL. They produce the safe efficient plans with some kind of an Anti-Join.

LEFT JOIN is less efficient, since it makes no attempt to skip the already matched values in the right table, returning all results and filtering them out instead.
http://explainextended.com/2009/09/15/not-in-vs-not-exists-vs-left-join-is-null-sql-server/

Question 43. How to find the statistics are outdated?

Answer –

You can check statistics by using below command –


DBCC SHOW_STATISTICS('TestRIDInxs','Ix_Index')

Table Name – TestRIDInxs
Index Name - 'Ix_Index

Output of the above query is given below-

Question 44. How to find the query running on a given SPID?

Answer-

You can use below query to find query running on an active SPID. Number (52) is the SPID showing in the below example.


DECLARE @sqltext VARBINARY(128)
SELECT @sqltext = sql_handle
FROM sys.dm_exec_requests
WHERE session_id = 52
SELECT TEXT
FROM sys.dm_exec_sql_text(@sqltext);

Question 45. Is “TRUNCATE” DDL or DML command?

Answer- DDL

Question 46. I have written a recursive CTE, it’s been running infinitely and failed with an error. The requirement is that the recursion required for 1000 times. How could you be able to handle the situation?

Answer-

Well if you write a proper recursive CTE then it will not run indefinitely.

MAXRECURSION can be used to break the recursive CTE. MAXRECURSION can be used to prevent a poorly formed recursive CTE from entering into an infinite loop.

The following example intentionally creates an infinite loop and uses the MAXRECURSION hint to limit the number of recursion levels to ten.


;WITH CTE AS
(
	SELECT ParentID FROM Hierarchies WHERE ParentID = 1
	UNION ALL
	SELECT C.ParentID FROM Hierarchies H INNER JOIN CTE C
	ON C.ParentID = H.ParentID
)
SELECT * FROM CTE
OPTION (MAXRECURSION 10);
GO

You will also get this kind of message-

Msg 530, Level 16, State 1, Line 1
The statement terminated. The maximum recursion 10 has been exhausted before statement completion.

Question 47. Which is better a CTE or a subquery? Why?

Answer-

Both CTEs and Sub Queries have pretty much the same performance and function.

CTE’s have an advantage over using a subquery in that you can use recursion in a CTE.

The biggest advantage of using CTE is readability. CTEs can be reference multiple times in the same statement where as sub query cannot.

Question 48. Any alternative to triggers?

Answer –

There are some options available depending exclusively on the requirement. They are
Change Data Capture
Change Tracking
Using output clause (Write the data out to a specific Audit table )
Computed column

Question 49. Why resourceDB introduced?

Answer –

It is a read-only database that contains system objects that are included with SQL Server. SQL Server system objects, such as sys.objects, are physically persisted in the Resource database, but they logically appear in the sys schema of every database. The Resource database does not contain user data or user metadata.

Also Resource database makes upgrading to a new version of SQL Server an easier and faster procedure. In earlier versions of SQL Server, upgrading required dropping and creating system objects. Because the Resource database file contains all system objects, an upgrade is now accomplished simply by copying the single Resource database file to the local server.

For details please visit –

http://blogs.msdn.com/b/vsanil/archive/2012/11/02/resource-database-common-questions.aspx

Question 50. How to create linked server.

Answer –

Please use below T-SQL script or follow the screen shots


USE [master]
GO
EXEC MASTER.dbo.sp_addlinkedserver
    @server = N'SRVR002\ACCTG',
    @srvproduct=N'SQL Server' ;
GO

SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers) [Page – 4]

15 Friday May 2015

Posted by Pawan Kumar Khowal in Download SQL Interview Q's, SQL Server Interview Questions

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SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers) Download – Pawan Kumar Khowal – SQL SERVER Interview Questions with Answers

PAGE – 4

Question 31- I have a table with millions of rows. I want to retain only last 5% of the rows? How does u do it?

Answer –

Transfer last 5 % rows in a temp table 2. Delete all the rows from the table. 3. Move back data from temp table to the main table.

Question 32 – Explain transaction log in detail

Answer –

A transaction log stores every transaction made to a SQL Server database, except some which are minimally logged like BULK IMPORT or SELECT INTO. Internally it is split into the smaller parts called Virtual Log Files (VLFs). When one VLF becomes full logging continue to write into the next available in the transaction log. The transaction log file can be represented as a circular file. When the logging reaches the end of the file it starts again from the beginning, but only if all the requirements has been met and the inactive parts has been truncated. The truncation process is necessary to mark all inactive parts so they can be used again and overwritten. A Log Sequence Number (LSN) identifies every transaction in the transaction log.

Question 33. Can we call a procedure from a function?

Answer – No.

Question 34. Can we write DML inside a function?

Answer – You can only use SELECT inside a function. You cannot use Insert, Update and Delete inside a function.

Question 35. How to avoid bookmark lookup in execution plan?

Answer-

Key lookup operation occurs when index seek is done on a non-clustered index to locate one or more rows, but the non-clustered index does not contain all the columns necessary for the query. The clustered index key (which is always included in all non-clustered indexes) is then used to locate the row in the clustered index, to retrieve the remaining data. We can include all the necessary columns in the NonClustered key.

Question 36. What is XACT_ABORT ON?

Answer – When SET XACT_ABORT is ON, if a Transact-SQL statement raises a run-time error, the entire transaction is terminated and rolled back. When SET XACT_ABORT is OFF, in some cases only the Transact-SQL statement that raised the error is rolled back and the transaction continues processing. Depending upon the severity of the error, the entire transaction may be rolled back even when SET XACT_ABORT is OFF. OFF is the default setting. Compile errors, such as syntax errors, are not affected by SET XACT_ABORT. XACT_ABORT must be set ON for data modification statements in an implicit or explicit transaction against most OLE DB providers, including SQL Server. The only case where this option is not required is if the provider supports nested transactions. Sample e.g.

--

SET XACT_ABORT OFF;
SET XACT_ABORT ON;

--

Question 37. Does temp tables and table variables both stored in tempDB?

Answer-

Temp tables and table variables are both objects created INSIDE TEMPDB. They are not created in memory. Their presence in memory is a result of their usage. Meaning: their pages are transferred and stored inside the buffer pool because this is the common way SQL Server works with all data pages – when a data page is requested for reads or writes it is read from disk and saved in the Buffer pool. So unless they are too big or server memory is under pressure they will be cached in memory anyway. For details please visit – http://maginaumova.com/temp-tables-or-temp-variables/

Question 38 – Explain Column Store Index

Answer- It’s a new feature introduced in SQL 2012. The column store index stores data in columnar fashion and uses compression aggressively to reduce the disk I/O needed to serve the query request. A ColumnStore index stores each column in a separate set of disk pages, rather than storing multiple rows per page as data traditionally has been stored. We can only create non clustered ColumnStore index. Column store index has been designed to substantially accelerate common data warehouse queries, which require scanning, aggregation and filtering of large amounts of data or joining multiple tables like a star schema. With column store index, you can get interactive response time for queries against billions of rows on an economical SMP server with enough RAM to hold your frequently accessed data. Creating a ColumnStore index is a parallel operation, subject to the limitations on the number of CPUs available and any restrictions set on MAXDOP setting.

Question 39. What happens when a rollback happens in inside a nested stored procedure?

Answer- If the rollback is present in the outermost transaction then everything will be rolled back irrespective of what is written in the inner transactions. If the rollback is present in the inner transaction then we will not have any transaction in the outer most transaction. Also note that the output depends on the statements written. For details please visit – https://msbiskills.com/2015/05/12/sql-server-nested-transactions-rollback-or-commit-confusions/

Question 40. Why cursors are so costly?

Answer-

SQL Server like any good relational database management system (RDBMS) is optimized for set-based operations and NOT for row by row operation. Cursors will fetch data on a row by row basis, and it takes heck of a lot longer to do so. This is because the set-based logic for which RDBMS systems like SQL Server are optimized is completely broken and the entire query process has to be repeated for each row. For details please visit – http://www.sqlshack.com/sql-server-cursor-performance-problems/

SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers) [Page -3]

15 Friday May 2015

Posted by Pawan Kumar Khowal in Download SQL Interview Q's, SQL Server Interview Questions

≈ 3 Comments

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SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers)

Download – Pawan Kumar Khowal – SQL SERVER Interview Questions with Answers

PAGE – 3

Question 21. What are the magic tables? Do we have “Updated” magic table?

Answer-

No we don’t have UPDATED magic table.
The ‘magic tables’ are the INSERTED and DELETED tables, as well as the update() and columns_updated() functions, and are used to determine the changes resulting from DML statements.
• For an INSERT statement, the INSERTED table will contain the inserted rows.
• For an UPDATE statement, the INSERTED table will contain the rows after an update, and the DELETED table will contain the rows before an update.
• For a DELETE statement, the DELETED table will contain the rows to be deleted.

Question 22. What is a filtered index?

Answer-

Consider filter index as — > INDEX with a Where Clause

It uses a filter predicate to index a portion of rows in the table. A filtered index can improve query performance as well as reduce index maintenance and storage costs compared with full-table indexes.

Important points about filtered indexes are –
• They can be created only as NonClustered Index
• They can be used on Views only if they are persisted views.
• They cannot be created on full-text Indexes.

Please check out the sample example below –


CREATE NONCLUSTERED INDEX Ix_NCI ON TestRIDInxs(ID)
WHERE Title= 20

Question 23. Basic difference between stored procedure and user defined function?

Answer-

There are many differences between functions and stored procedures. Some of them are given below-

Functions will allow only Select statement, it will not allow us to use insert, update and delete statement.
Procedures can have select statements as well as DML statements such as insert, update, delete.

Transactions are not allowed within functions. Can use transactions within Stored procedures.

For details please visit –

https://msbiskills.com/2010/08/12/differences-between-stored-procedure-and-function-in-sql-server/

Question 24. See we have a simple query that’s calling a static function, like “Select * from employee where joiningdate < getstaticdate()”? Does it call function for every time or only for matched rows? How you tune this query?

Answer-

We can do something like below-


DECLARE @t AS DATETIME = GETDATE()
SELECT * FROM TestRIDInxs WHERE joiningDate <= @t

Question 25. Why should we use CTE?

Answer-

A CTE can be used:
• For recursion
• Substitute for a view when the general use of a view is not required; that is, you do not have to store the definition in metadata.
• Reference the resulting non large table multiple times in the same statement.

Question 26. What is the difference between sub query and correlated query

Answer-

Subquery: – The inner query is executed only once. The inner query will get executed first and the output of the inner query used by the outer query. The inner query is not dependent on outer query.

Please check out the example below-


SELECT * FROM Visits WHERE CustomerId IN (SELECT CustomerId FROM Visits WHERE VisitDate = '2013-09-03 00:00:00.000')

Correlated subquery: – The outer query will get executed first and for every row of outer query, inner query will get executed. So the inner query will get executed as many times as number of rows in result of the outer query. The outer query output can use the inner query output for comparison. This means inner query and outer query dependent on each other

Please check out the famous example below- ( 2nd Highest Salary )


SELECT * FROM NthHighest N WHERE 1 = (SELECT DISTINCT(COUNT(*)) FROM NthHighest m WHERE n.Salary < m.Salary )

Question 27 – How do you retrieve random 5 rows from a table

Answer –


SELECT TOP 5 * FROM [Table_Name] ORDER BY NEWID()

Question 28 – What exactly you check in the query execution plan window?

Answer –

Query Execution Plans describe the steps and the order used to access data in the database. Normally we check the execution plan for missing indexes, if any. We also check the execution plans if the query or stored procedure is working slowly. If working slowly we need to check for Red flags and see how we can remove them.

You can use execution plan to check out following things –

• Missing Indexes
• Missing Statistics
• Red Flags
• How data is being retrieved (Which path , what all indexes are used)

Question 29 – What is the output of SELECT Len(1234.56)

Answer – 7 (In this case it will consider all the characters including.)

Question 30 – Where in MS SQL Server is ’100’ equal to ‘0’?

Ans – Fill Factor is the answer. Fill-factor settings of 0 and 100 are equal. If fill-factor is set to 100 or 0, the SQL Server Engine fills pages to their capacity while creating indexes.
Server default Value = 0.

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SQL SERVER Interview Questions & Answers – SET 1 (50 Questions & Answers) [Page – 5]

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