SQL Interview Questions and Answers

Structured Query Language (SQL), pronounced "sequel", is a language that provides an interface to relational database systems. It was developed by IBM in the 1970s for use in System R. SQL is a de facto standard, as well as an ISO and ANSI standard

SQL Interviews are getting tough these days as the technology grows faster. To get through the SQL interview one needs to update him/herself in a regular manner. Having said that, just before the interview, it is very important to have a quick glance of the reputed SQL questions and answers to make yourself comfortable during the interview process. This is where DoAnswers.com helps you in renewing yourself on SQL and various other technologies interview preparation.

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1. As a part of your job, what are the DBCC commands that you commonly use for database maintenance?

DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC, DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there are a whole load of DBCC commands which are very useful for DBAs. Check out SQL Server books online for more information.

2. Can you have a nested transaction?

Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT

3. CREATE INDEX myIndex ON myTable(myColumn) What type of Index will get created after executing the above statement?

Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.

4. Define candidate key, alternate key, composite key.

A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys. A key formed by combining at least two or more columns is called composite key.

5. Explain Active/Active and Active/Passive cluster configurations

Hopefully you have experience setting up cluster servers. But if you don't, at least be familiar with the way clustering works and the two clusterning configurations Active/Active and Active/Passive. SQL Server books online has enough information on this topic and there is a good white paper available on Microsoft site.

6. Explain CREATE DATABASE syntax

Many of us are used to craeting databases from the Enterprise Manager or by just issuing the command: CREATE DATABAE MyDB. But what if you have to create a database with two filegroups, one on drive C and the other on drive D with log on drive E with an initial size of 600 MB and with a growth factor of 15%? That's why being a DBA you should be familiar with the CREATE DATABASE syntax. Check out SQL Server books online for more information.

7. Explain different isolation levels

An isolation level determines the degree of isolation of data between concurrent transactions. The efault SQL Server solation level is Read Committed. Here are the other isolation levels (in the ascending order of isolation): Read Uncommitted, Read Committed,Repeatable Read, Serializable. See SQL Server books online for an explanation of the isolation levels. Be sure to read about SET TRANSACTION ISOLATION LEVEL, which lets you customize the isolation level at the connection level.

8. Explain the architecture of SQL Server

This is a very important question and you better be able to answer it if consider yourself a DBA. SQL Server books online is the best place to read about SQL Server architecture. Read up the chapter dedicated to SQL Server Architecture.

9. Explain the storage models of OLAP

Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more infomation.

10. Explian different types of BACKUPs avaialabe in SQL Server? Given a particular scenario, how would you go about choosing a backup plan?

Types of backups you can create in SQL Sever 7.0+ are Full database backup, differential database backup, transaction log backup, filegroup backup. Check out the BACKUP and RESTORE commands in SQL Server books online.Be prepared to write the commands in your interview. Books online also has information on detailed backup/restore architecture and when one should go for a particular kind of backup.

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