10 Apr, 2007
Database Interview Questions
- What is a deadlock and what is a live lock? How will you go about resolving deadlocks?
- Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other’s piece. Each process would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user’s process. A livelock is one, where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely. Check out SET DEADLOCK_PRIORITY and “Minimizing Deadlocks” in SQL Server books online.
- What is blocking and how would you troubleshoot it?
- Blocking happens when one connection from an application holds a lock and a second connection requires a conflicting lock type. This forces the second connection to wait, blocked on the first. Read up the following topics in SQL Server books online: Understanding and avoiding blocking, Coding efficient transactions. Explain CREATE DATABASE syntax Many of us are used to creating 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.
- How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode?
- SQL Server can be started from command line, using the SQLSERVR.EXE. This EXE has some very important parameters with which a DBA should be familiar with. -m is used for starting SQL Server in single user mode and -f is used to start the SQL Server in minimal configuration mode. Check out SQL Server books online for more parameters and their explanations.
- 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.
- What are statistics, under what circumstances they go out of date, how do you update them?
- Statistics determine the selectivity of the indexes. If an indexed column has unique values then the selectivity of that index is more, as opposed to an index with non-unique values. Query optimizer uses these indexes in determining whether to choose an index or not while executing a query. Some situations under which you should update statistics: 1) If there is significant change in the key values in the index 2) If a large amount of data in an indexed column has been added, changed, or removed (that is, if the distribution of key values has changed), or the table has been truncated using the TRUNCATE TABLE statement and then repopulated 3) Database is upgraded from a previous version. Look up SQL Server books online for the following commands: UPDATE STATISTICS, STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS, DROP STATISTICS, sp_autostats, sp_createstats, sp_updatestats
- What are the different ways of moving data/databases between servers and databases in SQL Server?
- There are lots of options available, you have to choose your option depending upon your requirements. Some of the options you have are: BACKUP/RESTORE, dettaching and attaching databases, replication, DTS, BCP, logshipping, INSERT…SELECT, SELECT…INTO, creating INSERT scripts to generate data.
- Explain 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.
- What is database replication? What are the different types of replication you can set up in SQL Server?
- Replication is the process of copying/moving data between databases on the same or different servers. SQL Server supports the following types of replication scenarios: � Snapshot replication � Transactional replication (with immediate updating subscribers, with queued updating subscribers) � Merge replication See SQL Server books online for indepth coverage on replication. Be prepared to explain how different replication agents function, what are the main system tables used in replication etc.
- How to determine the service pack currently installed on SQL Server?
- The global variable @@Version stores the build number of the sqlservr.exe, which is used to determine the service pack installed. To know more about this process visit SQL Server service packs and versions.
