SQL Interview Questions with Answers

What is normalization? Explain different levels of normalization?
Check out the article Q100139 from Microsoft knowledge base and ofcourse, there's much more information available in the net. It'll be agood 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 okayif you can explain till third normal form.
What is denormalization and when would you go for it?
As the name indicates, denormalization is the reverse process ofnormalization. It's the controlled introduction of redundancy in tothe database design. It helps improve the query performance as thenumber of joins could be reduced.
How do you implement one-to-one, one-to-many and many-to- anyrelationships while designing tables?
One-to-One relationship can be implemented as a single table andrarely as two tables with primary and foreign key relationships.One-to-Many relationships are implemented by splitting the data intotwo tables with primary key and foreign key relationships.Many-to-Many relationships are implemented using a junction table withthe keys from both the tables forming the composite primary key of thejunction table.
It will be a good idea to read up a database designing fundamentalstext book.
What's the difference between a primary key and a unique key?
Both primary key and unique enforce uniqueness of the column on whichthey are defined. But by default primary key creates a clustered indexon the column, where are unique creates a nonclustered index bydefault. Another major difference is that, primary key doesn't allowNULLs, but unique key allows one NULL only.
What are user defined datatypes and when you should go for them?
User defined datatypes let you extend the base SQL Server datatypes byproviding a descriptive name, and format to the database. Take forexample, in your database, there is a column called Flight_Num whichappears in many tables. In all these tables it should be varchar(8).In this case you could create a user defined datatype calledFlight_num_type of varchar(8) and use it across all your tables.
See sp_addtype, sp_droptype in books online.
What is bit datatype and what's the information that can be storedinside a bit column?
Bit datatype is used to store boolean information like 1 or 0 (true orfalse). Untill SQL Server 6.5 bit datatype could hold either a 1 or 0and there was no support for NULL. But from SQL Server 7.0 onwards,bit datatype can represent a third state, which is NULL.
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 thetable has more than one candidate key, one of them will become theprimary key, and the rest are called alternate keys.
A key formed by combining at least two or more columns is calledcomposite key.
What are defaults? Is there a column to which a default can't be bound?
A default is a value that will be used by a column, if no value issupplied to that column while inserting data. IDENTITY columns andtimestamp columns can't have defaults bound to them. See CREATEDEFUALT in books online.
Back to topSQL Server architecture (top)
What is a transaction and what are ACID properties?
A transaction is a logical unit of work in which, all the steps mustbe performed or none. ACID stands for Atomicity, Consistency,Isolation, Durability. These are the properties of a transaction. Formore information and explanation of these properties, see SQL Serverbooks online or any RDBMS fundamentals text book.
Explain different isolation levels
An isolation level determines the degree of isolation of data betweenconcurrent transactions. The default SQL Server isolation level isRead Committed. Here are the other isolation levels (in the ascendingorder of isolation): Read Uncommitted, Read Committed, RepeatableRead, Serializable. See SQL Server books online for an explanation ofthe isolation levels. Be sure to read about SET TRANSACTION ISOLATIONLEVEL, which lets you customize the isolation level at the connectionlevel.
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 clusteredindex gets created on the primary key, unless specified otherwise.
What's the maximum size of a row?
8060 bytes. Don't be surprised with questions like 'what is themaximum number of columns per table'. Check out SQL Server booksonline for the page titled: "Maximum Capacity Specifications".
Explain Active/Active and Active/Passive cluster configurations
Hopefully you have experience setting up cluster servers. But if youdon't, at least be familiar with the way clustering works and the twoclusterning configurations Active/Active and Active/Passive. SQLServer books online has enough information on this topic and there isa good white paper available on Microsoft site.
Explain the architecture of SQL Server
This is a very important question and you better be able to answer itif consider yourself a DBA. SQL Server books online is the best placeto read about SQL Server architecture. Read up the chapter dedicatedto SQL Server Architecture.
What is lock escalation?
Lock escalation is the process of converting a lot of low level locks(like row locks, page locks) into higher level locks (like tablelocks). 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-grainlocks. Lock escalation threshold was definable in SQL Server 6.5, butfrom SQL Server 7.0 onwards it's dynamically managed by SQL Server.
What's the difference between DELETE TABLE and TRUNCATE TABLE commands?
DELETE TABLE is a logged operation, so the deletion of each row getslogged in the transaction log, which makes it slow. TRUNCATE TABLEalso deletes all the rows in a table, but it won't log the deletion ofeach row, instead it logs the deallocation of the data pages of thetable, which makes it faster. Of course, TRUNCATE TABLE can be rolledback.
Explain the storage models of OLAP
Check out MOLAP, ROLAP and HOLAP in SQL Server books online for moreinfomation.
What are the new features introduced in SQL Server 2000 (or the latestrelease of SQL Server at the time of your interview)? What changedbetween the previous version of SQL Server and the current version?
This question is generally asked to see how current is your knowledge.Generally there is a section in the beginning of the books onlinetitled "What's New", which has all such information. Of course,reading just that is not enough, you should have tried those things tobetter answer the questions. Also check out the section titled"Backward Compatibility" in books online which talks about the changesthat have taken place in the new version.
What are constraints? Explain different types of constraints.
Constraints enable the RDBMS enforce the integrity of the databaseautomatically, without needing you to create triggers, rule or defaults.
Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY
For an explanation of these constraints see books online for the pagestitled: "Constraints" and "CREATE TABLE", "ALTER TABLE"
Whar is an index? What are the types of indexes? How many clusteredindexes can be created on a table? I create a separate index on eachcolumn of a table. what are the advantages and disadvantages of thisapproach?
Indexes in SQL Server are similar to the indexes in books. They helpSQL Server retrieve the data quicker.
Indexes are of two types. Clustered indexes and non-clustered indexes.When you craete a clustered index on a table, all the rows in thetable are stored in the order of the clustered index key. So, therecan be only one clustered index per table. Non-clustered indexes havetheir own storage separate from the table data storage. Non-clusteredindexes are stored as B-tree structures (so do clustered indexes),with the leaf level nodes having the index key and it's row locater.The row located could be the RID or the Clustered index key, dependingup on the absence or presence of clustered index on the table.
If you create an index on each column of a table, it improves thequery performance, as the query optimizer can choose from all theexisting indexes to come up with an efficient execution plan. At thesame t ime, data modification operations (such as INSERT, UPDATE,DELETE) will become slow, as every time data changes in the table, allthe indexes need to be updated. Another disadvantage is that, indexesneed disk space, the more indexes you have, more disk space is used.
Back to topDatabase administration (top)
What is RAID and what are different types of RAID configurations?
RAID stands for Redundant Array of Inexpensive Disks, used to providefault tolerance to database servers. There are six RAID levels 0through 5 offering different levels of performance, fault tolerance.MSDN has some information about RAID levels and for detailedinformation, check out the RAID advisory board's homepage
What are the steps you will take to improve performance of a poorperforming query?
This is a very open ended question and there could be a lot of reasonsbehind the poor performance of a query. But some general issues thatyou could talk about would be: No indexes, table scans, missing or outof date statistics, blocking, excess recompilations of storedprocedures, procedures and triggers without SET NOCOUNT ON, poorlywritten query with unnecessarily complicated joins, too muchnormalization, excess usage of cursors and temporary tables.
Some of the tools/ways that help you troubleshooting performanceproblems are: SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SETSTATISTICS IO ON, SQL Server Profiler, Windows NT /2000 Performancemonitor, Graphical execution plan in Query Analyzer.
Download the white paper on performance tuning SQL Server fromMicrosoft web site. Don't forget to check out sql-server-performance.com
What are the steps you will take, if you are tasked with securing anSQL Server?
Again this is another open ended question. Here are some things youcould talk about: Preferring NT authentication, using server, databseand application roles to control access to the data, securing thephysical database files using NTFS permissions, using an unguessableSA password, restricting physical access to the SQL Server, renamingthe Administrator account on the SQL Server computer, disabling theGuest account, enabling auditing, using multiprotocol encryption,setting up SSL, setting up firewalls, isolating SQL Server from theweb server etc.
Read the white paper on SQL Server security from Microsoft website.Also check out My SQL Server security best practices
What is a deadlock and what is a live lock? How will you go aboutresolving deadlocks?
Deadlock is a situation when two processes, each having a lock on onepiece of data, attempt to acquire a lock on the other's piece. Eachprocess would wait indefinitely for the other to release the lock,unless one of the user processes is terminated. SQL Server detectsdeadlocks and terminates one user's process.
A livelock is one, where a request for an exclusive lock isrepeatedly denied because a series of overlapping shared locks keepsinterfering. SQL Server detects the situation after four denials andrefuses further shared locks. A livelock also occurs when readtransactions monopolize a table or page, forcing a write transactionto wait indefinitely.
Check out SET DEADLOCK_PRIORITY and "Minimizing Deadlocks" in SQLServer books online. Also check out the article Q169960 from Microsoftknowledge base.
What is blocking and how would you troubleshoot it?
Blocking happens when one connection from an application holds a lockand a second connection requires a conflicting lock type. This forcesthe second connection to wait, blocked on the first.
Read up the following topics in SQL Server books online: Understandingand avoiding blocking, Coding efficient transactions.
Explain CREATE DATABASE syntax
Many of us are used to craeting databases from the Enterprise Manageror by just issuing the command: CREATE DATABAE MyDB. But what if youhave to create a database with two filegroups, one on drive C and theother on drive D with log on drive E with an initial size of 600 MBand with a growth factor of 15%? That's why being a DBA you should befamiliar with the CREATE DATABASE syntax. Check out SQL Server booksonline for more information.
How to restart SQL Server in single user mode? How to start SQL Serverin 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 befamiliar with. -m is used for starting SQL Server in single user modeand -f is used to start the SQL Server in minimal confuguration mode.Check out SQL Server books online for more parameters and theirexplanations.
As a part of your job, what are the DBCC commands that you commonlyuse for database maintenance?
DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC,DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But thereare 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, howdo you update them?
Statistics determine the selectivity of the indexes. If an indexedcolumn has unique values then the selectivity of that index is more,as opposed to an index with non-unique values. Query optimizer usesthese indexes in determining whether to choose an index or not whileexecuting a query.
Some situations under which you should update statistics:1) If there is significant change in the key values in the index2) If a large amount of data in an indexed column has been added,changed, or removed (that is, if the distribution of key values haschanged), or the table has been truncated using the TRUNCATE TABLEstatement and then repopulated3) Database is upgraded from a previous version
Look up SQL Server books online for the following commands: UPDATESTATISTICS, STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS, DROPSTATISTICS, sp_autostats, sp_createstats, sp_updatestats
What are the different ways of moving data/databases between serversand databases in SQL Server?
There are lots of options available, you have to choose your optiondepending 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 INSERTscripts to generate data.
Explian different types of BACKUPs avaialabe in SQL Server? Given aparticular scenario, how would you go about choosing a backup plan?
Types of backups you can create in SQL Sever 7.0+ are Full databasebackup, differential database backup, transaction log backup,filegroup backup. Check out the BACKUP and RESTORE commands in SQLServer books online. Be prepared to write the commands in yourinterview. Books online also has information on detailedbackup/restore architecture and when one should go for a particularkind of backup.
What is database replicaion? What are the different types ofreplication you can set up in SQL Server?
Replication is the process of copying/moving data between databases onthe same or different servers. SQL Server supports the following typesof 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. Beprepared to explain how different replication agents function, whatare 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 thesqlservr.exe, which is used to determine the service pack installed.To know more about this process visit SQL Server service packs andversions.
Back to topDatabase programming (top)
What are cursors? Explain different types of cursors. What are thedisadvantages of cursors? How can you avoid cursors?
Cursors allow row-by-row prcessing of the resultsets.
Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. Seebooks online for more information.
Disadvantages of cursors: Each time you fetch a row from the cursor,it results in a network roundtrip, where as a normal SELECT querymakes only one rowundtrip, however large the resultset is. Cursors arealso costly because they require more resources and temporary storage(results in more IO operations). Furthere, there are restrictions onthe SELECT statements that can be used with some types of cursors.
Most of the times, set based operations can be used instead ofcursors. Here is an example:
If you have to give a flat hike to your employees using the followingcriteria:
Salary between 30000 and 40000 -- 5000 hikeSalary between 40000 and 55000 -- 7000 hikeSalary between 55000 and 65000 -- 9000 hike
In this situation many developers tend to use a cursor, determine eachemployee's salary and update his salary according to the aboveformula. But the same can be achieved by multiple update statements orcan be combined in a single UPDATE statement as shown below:
UPDATE tbl_emp SET salary =CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000END
Another situation in which developers tend to use cursors: You need tocall a stored procedure when a column in a particular row meetscertain condition. You don't have to use cursors for this. This can beachieved using WHILE loop, as long as there is a unique key toidentify each row. For examples of using WHILE loop for row by rowprocessing, check out the 'My code library' section of my site orsearch for WHILE.
Write down the general syntax for a SELECT statements covering all theoptions.
Here's the basic syntax: (Also checkout SELECT in books online foradvanced syntax).
SELECT select_list[INTO new_table_]FROM table_source[WHERE search_condition][GROUP BY group_by__expression][HAVING search_condition][ORDER BY order__expression [ASC DESC] ]
What is a join and explain different types of joins.
Joins are used in queries to explain how different tables are related.Joins also let you select data from a table depending upon data fromanother table.
Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs arefurther classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULLOUTER JOINS.
For more information see pages from books online titled: "JoinFundamentals" and "Using Joins".
Can you have a nested transaction?
Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and@@TRANCOUNT
What is an extended stored procedure? Can you instantiate a COM objectby using T-SQL?
An extended stored procedure is a function within a DLL (written in aprogramming language like C, C++ using Open Data Services (ODS) API)that can be called from T-SQL, just the way we call normal storedprocedures using the EXEC statement. See books online to learn how tocreate extended stored procedures and how to add them to SQL Server.
Yes, you can instantiate a COM (written in languages like VB, VC++)object from T-SQL by using sp_OACreate stored procedure. Also seebooks online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty,sp_OADestroy. For an example of creating a COM object in VB andcalling it from T-SQL, see 'My code library' section of this site.
What is the system function to get the current user's user id?
USER_ID(). Also check out other system functions like USER_NAME(),SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().
What are triggers? How many triggers you can have on a table? How toinvoke a trigger on demand?
Triggers are special kind of stored procedures that get executedautomatically when an INSERT, UPDATE or DELETE operation takes placeon a table.
In SQL Server 6.5 you could define only 3 triggers per table, one forINSERT, one for UPDATE and one for DELETE. From SQL Server 7.0onwards, this restriction is gone, and you could create multipletriggers per each action. But in 7.0 there's no way to control theorder in which the triggers fire. In SQL Server 2000 you could specifywhich trigger fires first or fires last using sp_settriggerorder
Triggers can't be invoked on demand. They get triggered only when anassociated action (INSERT, UPDATE, DELETE) happens on the table onwhich they are defined.
Triggers are generally used to implement business rules, auditing.Triggers can also be used to extend the referential integrity checks,but wherever possible, use constraints for this purpose, instead oftriggers, as constraints are much faster.
Till SQL Server 7.0, triggers fire only after the data modificationoperation happens. So in a way, they are called post triggers. But inSQL Server 2000 you could create pre triggers also. Search SQL Server2000 books online for INSTEAD OF triggers.
Also check out books online for 'inserted table', 'deleted table' andCOLUMNS_UPDATED()
There is a trigger defined for INSERT operations on a table, in anOLTP system. The trigger is written to instantiate a COM object andpass the newly insterted rows to it for some custom processing. Whatdo you think of this implementation? Can this be implemented better?
Instantiating COM objects is a time consuming process and since youare doing it from within a trigger, it slows down the data insertionprocess. Same is the case with sending emails from triggers. Thisscenario can be better implemented by logging all the necessary datainto a separate table, and have a job which periodically checks thistable and does the needful.
What is a self join? Explain it with an example.
Self join is just like any other join, except that two instances ofthe same table will be joined in the query. Here is an example:Employees table which contains rows for normal employees as well asmanagers. So, to find out the managers of all the employees, you needa self join.
CREATE TABLE emp(empid int,mgrid int,empname char(10))
INSERT emp SELECT 1,2,'Vyas'INSERT emp SELECT 2,3,'Mohan'INSERT emp SELECT 3,NULL,'Shobha'INSERT emp SELECT 4,2,'Shridhar'INSERT emp SELECT 5,2,'Sourabh'
SELECT t1.empname [Employee], t2.empname [Manager]FROM emp t1, emp t2WHERE t1.mgrid = t2.empid
Here's an advanced query using a LEFT OUTER JOIN that even returns theemployees without managers (super bosses)
SELECT t1.empname [Employee], COALESCE(t2.empname, 'No manager') [Manager]FROM emp t1LEFT OUTER JOINemp t2ONt1.mgrid = t2.empid