SQL Server interview questions
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Here are some SQL Server DBA/Developer interview questions I faced myself personally and/or heard from people. I will try to answer these questions briefly here, but be advised that these answers may not be complete and it will be better for you to go through text books, books online and other resources on the net. At the end of this article, I listed some useful books for aspiring DBAs and developers.
Before you go for the interview, be prepared to explain the database design of one of your latest projects. Don't be surprised if the interviewer asks you to draw ER diagrams.
Well, here are some questions for you. Hope this helps you prepare for your interview. Wish you all the best in your job hunt! Feel free to email me 'interview questions' that you've faced.
Questions are categorized under the following sections, for your convenience:
Check out the article Q100139 from Microsoft knowledge base and of course, there's much more information available in the net. It'll be a good idea to get a hold of any RDBMS fundamentals text book, especially the one by C. J. Date. Most of the times, it will be okay if you can explain till third normal form.
As the name indicates, denormalization is the reverse process of normalization. It's the controlled introduction of redundancy in to the database design. It helps improve the query performance as the number of joins could be reduced.
One-to-One relationship can be
implemented as a single table and rarely as two tables
with primary and foreign key relationships.
Both primary key and unique enforce 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.
defined datatypes let you extend the base SQL Server
datatypes by providing a descriptive name, and format to
the database. Take for example, in your database, there
is a column called Flight_Num which appears in many
tables. In all these tables it should be varchar(8). In
this case you could create a user defined datatype called
Flight_num_type of varchar(8) and use it across all your
Bit datatype is used to store boolean information like 1 or 0 (true or false). Untill SQL Server 6.5 bit datatype could hold either a 1 or 0 and there was no support for NULL. But from SQL Server 7.0 onwards, bit datatype can represent a third state, which is NULL.
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 default is a value that will be used by a column, if no value is supplied to that column while inserting data. IDENTITY columns and timestamp columns can't have defaults bound to them. See CREATE DEFUALT in books online.
A transaction is a logical unit of work in which, all the steps must be performed or none. ACID stands for Atomicity, Consistency, Isolation, Durability. These are the properties of a transaction. For more information and explanation of these properties, see SQL Server books online or any RDBMS fundamentals text book.
An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation 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.
Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.
8060 bytes. Don't be surprised with questions like 'what is the maximum number of columns per table'. Check out SQL Server books online for the page titled: "Maximum Capacity Specifications".
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.
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.
Lock escalation is the process of converting a lot of low level locks (like row locks, page locks) into higher level locks (like table locks). Every lock is a memory structure too many locks would mean, more memory being occupied by locks. To prevent this from happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks. Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards it's dynamically managed by SQL Server.
DELETE TABLE is a logged operation, so the deletion of each row gets logged in the transaction log, which makes it slow. TRUNCATE TABLE also deletes all the rows in a table, but it won't log the deletion of each row, instead it logs the deallocation of the data pages of the table, which makes it faster. Of course, TRUNCATE TABLE can be rolled back.
Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more infomation.
This question is generally asked to see how current is your knowledge. Generally there is a section in the beginning of the books online titled "What's New", which has all such information. Of course, reading just that is not enough, you should have tried those things to better answer the questions. Also check out the section titled "Backward Compatibility" in books online which talks about the changes that have taken place in the new version.
enable the RDBMS enforce the integrity of the database
automatically, without needing you to create triggers,
rule or defaults.
SQL Server are similar to the indexes in books. They help
SQL Server retrieve the data quicker.
RAID stands for Redundant Array of Inexpensive Disks, used to provide fault tolerance to database servers. There are six RAID levels 0 through 5 offering different levels of performance, fault tolerance. MSDN has some information about RAID levels and for detailed information, check out the RAID advisory board's homepage
This is a very open ended question and
there could be a lot of reasons behind the poor
performance of a query. But some general issues that you
could talk about would be: No indexes, table scans,
missing or out of date statistics, blocking, excess
recompilations of stored procedures, procedures and
triggers without SET NOCOUNT ON, poorly written query
with unnecessarily complicated joins, too much
normalization, excess usage of cursors and temporary
Again this is another open ended
question. Here are some things you could talk about:
Preferring NT authentication, using server, databse and
application roles to control access to the data, securing
the physical database files using NTFS permissions, using
an unguessable SA password, restricting physical access
to the SQL Server, renaming the Administrator account on
the SQL Server computer, disabling the Guest account,
enabling auditing, using multiprotocol encryption,
setting up SSL, setting up firewalls, isolating SQL
Server from the web server etc.
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.
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.
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.
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 confuguration mode. Check out SQL Server books online for more parameters and their explanations.
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.
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
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.
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.
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:
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.
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.
allow row-by-row prcessing of the resultsets.
basic syntax: (Also checkout SELECT in books online for
used in queries to explain how different tables are
related. Joins also let you select data from a table
depending upon data from another table.
Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
extended stored procedure is a function within a DLL
(written in a programming language like C, C++ using Open
Data Services (ODS) API) that can be called from T-SQL,
just the way we call normal stored procedures using the
EXEC statement. See books online to learn how to create
extended stored procedures and how to add them to SQL
USER_ID(). Also check out other system functions like USER_NAME(), SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().
are special kind of stored procedures that get executed
automatically when an INSERT, UPDATE or DELETE operation
takes place on a table.
Instantiating COM objects is a time consuming process and since you are doing it from within a trigger, it slows down the data insertion process. Same is the case with sending emails from triggers. This scenario can be better implemented by logging all the necessary data into a separate table, and have a job which periodically checks this table and does the needful.
is just like any other join, except that two instances of
the same table will be joined in the query. Here is an
example: Employees table which contains rows for normal
employees as well as managers. So, to find out the
managers of all the employees, you need a self join.
Here's an advanced query using a LEFT OUTER JOIN
that even returns the employees without managers (super bosses)
For a solution see the 'My code library' section of this site.Suggested reading for aspiring SQL Server DBAs and developers: