SQL Server Interview Questions and Answers – Part 6

SQL Server Interview Questions and Answers
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UPDATE : Interview Questions and Answers are now updated with SQL Server 2008 Questions and its answers. New Location : SQL Server 2008 Interview Questions and Answers.

What are the properties of the Relational tables?
Relational tables have six properties:

  • Values are atomic.
  • Column values are of the same kind.
  • Each row is unique.
  • The sequence of columns is insignificant.
  • The sequence of rows is insignificant.
  • Each column must have a unique name.

What is De-normalization?
De-normalization is the process of attempting to optimize the performance of a database by adding redundant data. It is sometimes necessary because current DBMSs implement the relational model poorly. A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De-normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access.

How to get @@error and @@rowcount at the same time?
If @@Rowcount is checked after Error checking statement then it will have 0 as the value of @@Recordcount as it would have been reset.
And if @@Recordcount is checked before the error-checking statement then @@Error would get reset. To get @@error and @@rowcount at the same time do both in same statement and store them in local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR

What is Identity?
Identity (or AutoNumber) is a column that automatically generates numeric values. A start and increment value can be set, but most DBA leave these at 1. A GUID column also generates numbers, the value of this cannot be controled. Identity/GUID columns do not need to be indexed.

What is a Scheduled Jobs or What is a Scheduled Tasks?
Scheduled tasks let user automate processes that run on regular or predictable cycles. User can schedule administrative tasks, such as cube processing, to run during times of slow business activity. User can also determine the order in which tasks run by creating job steps within a SQL Server Agent job. E.g. Back up database, Update Stats of Tables. Job steps give user control over flow of execution.  If one job fails, user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.

What is a table called, if it does not have neither Cluster nor Non-cluster Index? What is it used for?
Unindexed table or Heap. Microsoft Press Books and Book On Line (BOL) refers it as Heap.
A heap is a table that does not have a clustered index and, therefore, the pages are not linked by pointers. The IAM pages are the only structures that link the pages in a table together.
Unindexed tables are good for fast storing of data. Many times it is better to drop all indexes from table and than do bulk of inserts and to restore those indexes after that.

What is BCP? When does it used?
BulkCopy is a tool used to copy huge amount of data from tables and views. BCP does not copy the structures same as source to destination.

How do you load large data to the SQL server database?
BulkCopy is a tool used to copy huge amount of data from tables. BULK INSERT command helps to Imports a data file into a database table or view in a user-specified format.

Can we rewrite subqueries into simple select statements or with joins?
Subqueries can often be re-written to use a standard outer join, resulting in faster performance. As we may know, an outer join uses the plus sign (+) operator to tell the database to return all non-matching rows with NULL values. Hence we combine the outer join with a NULL test in the WHERE clause to reproduce the result set without using a sub-query.

Can SQL Servers linked to other servers like Oracle?
SQL Server can be lined to any server provided it has OLE-DB provider from Microsoft to allow a link. E.g. Oracle has a OLE-DB provider for oracle that Microsoft provides to add it as linked server to SQL Server group.

How to know which index a table is using?
SELECT table_name,index_name FROM user_constraints

How to copy the tables, schema and views from one SQL server to another?
Microsoft SQL Server 2000 Data Transformation Services (DTS) is a set of graphical tools and programmable objects that lets user extract, transform, and consolidate data from disparate sources into single or multiple destinations.

What is Self Join?
This is a particular case when one table joins to itself, with one or two aliases to avoid confusion. A self join can be of any type, as long as the joined tables are the same. A self join is rather unique in that it involves a relationship with only one table. The common example is when company have a hierarchal reporting structure whereby one member of staff reports to another.

What is Cross Join?
A cross join that does not have a WHERE clause produces the Cartesian product of the tables involved in the join. The size of a Cartesian product result set is the number of rows in the first table multiplied by the number of rows in the second table. The common example is when company wants to combine each product with a pricing table to analyze each product at each price.

Which virtual table does a trigger use?
Inserted and Deleted.

List few advantages of Stored Procedure.

  • Stored procedure can reduced network traffic and latency, boosting application performance.
  • Stored procedure execution plans can be reused, staying cached in SQL Server’s memory, reducing server overhead.
  • Stored procedures help promote code reuse.
  • Stored procedures can encapsulate logic. You can change stored procedure code without affecting clients.
  • Stored procedures provide better security to your data.

What is DataWarehousing?

  • Subject-oriented, meaning that the data in the database is organized so that all the data elements relating to the same real-world event or object are linked together;
  • Time-variant, meaning that the changes to the data in the database are tracked and recorded so that reports can be produced showing changes over time;
  • Non-volatile, meaning that data in the database is never over-written or deleted, once committed, the data is static, read-only, but retained for future reporting;
  • Integrated, meaning that the database contains data from most or all of an organization’s operational applications, and that this data is made consistent.

What is OLTP(OnLine Transaction Processing)?
In OLTP – online transaction processing systems relational database design use the discipline of data modeling and generally follow the Codd rules of data normalization in order to ensure absolute data integrity. Using these rules complex information is broken down into its most simple structures (a table) where all of the individual atomic level elements relate to each other and satisfy the normalization rules.

How do SQL server 2000 and XML linked? Can XML be used to access data?
FOR XML (ROW, AUTO, EXPLICIT)
You can execute SQL queries against existing relational databases to return results as XML rather than standard rowsets. These queries can be executed directly or from within stored procedures. To retrieve XML results, use the FOR XML clause of the SELECT statement and specify an XML mode of RAW, AUTO, or EXPLICIT.

OPENXML
OPENXML is a Transact-SQL keyword that provides a relational/rowset view over an in-memory XML document. OPENXML is a rowset provider similar to a table or a view. OPENXML provides a way to access XML data within the Transact-SQL context by transferring data from an XML document into the relational tables. Thus, OPENXML allows you to manage an XML document and its interaction with the relational environment.

What is an execution plan? When would you use it? How would you view the execution plan?
An execution plan is basically a road map that graphically or textually shows the data retrieval methods chosen by the SQL Server query optimizer for a stored procedure or ad-hoc query and is a very useful tool for a developer to understand the performance characteristics of a query or stored procedure since the plan is the one that SQL Server will place in its cache and use to execute the stored procedure or query. From within Query Analyzer is an option called “Show Execution Plan” (located on the Query drop-down menu). If this option is turned on it will display query execution plan in separate window when query is ran again.

Complete Series of SQL Server Interview Questions and Answers
SQL Server Interview Questions and Answers – Introduction
SQL Server Interview Questions and Answers – Part 1
SQL Server Interview Questions and Answers – Part 2
SQL Server Interview Questions and Answers – Part 3
SQL Server Interview Questions and Answers – Part 4
SQL Server Interview Questions and Answers – Part 5
SQL Server Interview Questions and Answers – Part 6
SQL Server Interview Questions and Answers Complete List Download

Reference : Pinal Dave (http://blog.SQLAuthority.com)

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84 thoughts on “SQL Server Interview Questions and Answers – Part 6

  1. I was given task to tranform the crystal report to sql server report format. There i have one problem. In crystal report, you can use the formula running total to evalulate and display the sequential row number of each items in a group result and also sequential row number for sub group as well. But in sql server report, I try to use the formula row_number but it only can display the number line 1 (start of the report file) and number xx (where xx is the end of the line in my report file) but that is totally wrong for my report requirement. I need a way that could help me only count the row number start from the group items (in table) instead of first line of the report file. Please help.. i have try so many times but still can’t work.

    Like

  2. As Per my opinion These Questions and answers are very helpful those who are in serious job Hunting.

    Many Many Thanks

    Like

  3. Hi ,
    I m Rupesh….
    I am facing a problem…
    I have a view which is related to many functions and tables
    and view is having over 6 lacs records and now I want to copy all these records to a table….

    I have tried all the options such as import /export and insert command also but it is taking so much time……..

    plz help its very urgent…

    Thanx inn advance

    Like

  4. Hi ,
    Just need a simple store procedure which deletes a record from 2 tables. The second delete is comparing file_name field. What is the correct syntax for the second delete statement. Any help will be much appreciated. Also is this possible?

    CREATE procedure up_test_delete(
    @test_id int
    )
    as
    delete from tests where test_id=@test_id
    delete from (another table) where file_name =’filename.aspx?test_id=’+@test_id

    Like

  5. Hi Pinal ,
    Thank you for your help! I really appreciate for it. Indeed you are helping me for my carreer. I am sure that you will help me in the problems and doubts that i will have in my job as i said i am a new SQL Server DBA and i do not know much but i am trying hard to learn new stuff . I need the docs for query optomization, query execution plan and query tunining. If you have steps for these please let me know.

    Thank you in advance.
    Abi

    Like

  6. hi!!!,
    this question answer is very good material for me and also it helps to anyone who want to get details info about sql server database.

    Like

  7. HI PINAL,
    I AM AMAR.

    I’VE A QUERY I WANT CREATE INSERT TRIGGER IN A TABLE IN SQL2005. AND ANOTHER QUERY IS I’VE TO DISPLAY
    THE TOTAL AMOUNT OF ALL THE SALES ORDERS ROUNDED OFF TO A WHOLE NUMBER. IN ADDITION , I’VE TO DISPLAY THE SALES ORDER ID AND THE TYPE OF CREDIT CARD THROUGH WHICH THE PAMENT WAS MODE.

    I’VE TO DISPLAY THE TOTAL AMOUNT DUE OF ALL THE ORDERS IN THE FOLLOWING TABULAR FORMAT :-

    ORDER VALUE
    THE TOTAL AMOUNT DUE FOR THE SALES ORDER ID:43659 I S $27231.55
    THE TOTAL AMOUNT DUE FOR THE SALES ORDER ID:43650 I S $1716.18

    PLEASE, HELP ME.

    Like

  8. Hi Pinal,
    What a beautiful question and answer. I am a interviewer I have been asking lot of question from your list.
    any way this very useful to new software eng.

    Thanks for your article.

    Regards
    Mahesh

    Like

  9. Hi Pinal,

    Hope u r good…
    I have some doubts on sql server
    How many maxium number of rows we can store in one table
    what is the minimum space occupied by a stored procedure

    Its urgent!!

    Thanx in advance…..

    bye

    Rupesh

    Like

  10. Hi Pinal,
    I have some doubts.
    1. There are 5 million rows in a table. I want to delet 2 million rows from the table. How can i do this?
    2. Homany .MDF files will be there in a Database if the database is of TBs size. ( As per me there will be just one MDF file but can we make/have more MDFs in other drives)

    I really appreciate for your help.

    Thanks
    Abi

    Like

  11. Hi Abi,

    Answers to your questions.
    1) I just use DELETE with WHERE clause with appropriate condition. It sometime takes more time however I use query hints WITH (nolock) to prevent any locking on tables.

    2) There will be only one MDF unless more than one MDF are created/added after database was created.

    Regards,
    Pinal Dave (SQLAuthority.com)

    Like

  12. Thank you Pinnal. You have been very helpful for all my queries. If i come to Las Vegas i will surely meet you.

    Thank you
    Abi

    Like

  13. Answer to Rupes’s Question No. 5

    You have to use OPENROWSET:

    example:
    Begin
    Set IDENTITY_INSERT dbo.Test1 ON
    Insert into Test1(Test_Id, Test_Nm)
    SELECT a.*
    FROM OPENROWSET(‘SQLNCLI’, ‘Server=(local);Trusted_Connection=yes;’,
    ‘SELECT test_Id, test_Nm
    FROM dbo.test’) AS a
    Set IDENTITY_INSERT dbo.Test1 OFF
    End

    Like

  14. Just an FYI…truncate statements can be rolled back inside the query execution process. I know you specified logs in your statement, but there is a lot of misinformation that truncate statement can’t be rolled back period.

    Furthremore, I don’t like the questions about the OPENROWSET and the case-sensitivity. Those open up a huge can of worms..and unless the interviewer knows what he is asking..he can end up looking real foolish.

    Otherwise, its a GREAT set of questions..very nice and concise…one of the BEST I have seen in a while from different sites.

    Like

  15. I really feel indebted to your effort.I take my hats off to your outstanding effort.Kudos to you.Your work is simply marvellous.It can help numerous of people like me.Continue enriching it with your experience and kowledge.
    BEST OF LUCK.

    Like

  16. Your website has been very helpful to me. Thank you for taking the time and effort to help other people. We really appreciate it. Take care.

    Like

  17. Thanks a Lot,
    It’s good to have these Questions organized at one place rather hunting for them in different sites.

    very helpful

    thanks again
    Deepak

    Like

  18. Thanks Pinal

    I have question

    I have 10 tables with each 500 million rows, now I have to merge all these tables in to one single table.

    Please let me know what is the fastest way for this.

    I used
    insert into tableB select * from table A (Its taking huge time)

    Please tell me any very good option available for this.

    Thanks
    Obula

    Like

  19. Hi All,
    i have a situation..

    i have need to partition a databse in sql server.i know the number of partitions to make and type of partitions .Now client asks for optimal hardware sizing for the partitions??

    Can anyone help….

    Tx in advance.

    Rgds
    Dillip

    Like

  20. HI Pinal,

    will you plz tell me that can we convert xml to sql using xquery and how ? Also tell me the another way of converting this?

    rgds
    seema

    Like

  21. Hi..

    Good questions but I think you mixes sql and oracle – or may be I am missing something

    regarding one of your question

    SELECT table_name,index_name FROM user_constraints ..

    If I am not wrong user_constraints is the table in Oracle. I do not see it on sql server. Am I missing anythign here? How do you find which index is in use in Sql server 2000?

    Thanks,

    Like

  22. Hi,

    Have you got any material on SQL Server 2005 Architecture Diagram with simple explanation and SQL Server Clustering ??

    Thanks,
    Brij.

    Like

  23. Tell me the ryt way if possible in sql server 2000 …
    I hav one table having 2 columns with many rows..
    i want to convert its rows into columns of another table and make dis one dynamically b,coz if more rows are added then more columns are creeated dynamically without using pivots…

    Like

  24. Sir ,

    I need yr help …………….
    Please sir get me solution of that question …………..

    where sqlserver store the information about tables and contraints and attributes and indexs

    Actual location …

    Thanks and regards
    Yaman Gupta

    Like

  25. Dear Sir,

    If you have SQL SERVER 2000 DBA and 2005 dba material, kindly mail me. I am certified in SQL SERVER. I have to update. Currently i don’t have material with me. If you have mail me.

    Suresh

    Like

  26. Dear Pinal

    In my last Telephonic interview, I was asked to desribed SQL Server Architecture, which is a very open ended subject as per BOL to be replied on Phone.
    Plz explain/describe a small & specific Definition for SQL Server Architecture.
    Regards
    SK

    Like

  27. Pinal,
    In sql 2K5, Is it possible to set a job to run every time it stops? We can set the interval to a small one so that it keeps checking if the job is running or not and then start it. But is there any better way?
    -Lakshmi

    Like

  28. Your answer for BCP gives me the perception that i can copy data only from sql server to a txt file. But bulk copy can be used the other way around too.

    Please make over the BCP answer if am right.

    I like the way you help others. All the best!

    Like

  29. I was shocked tos see all these question because all simlar questions came in test after my training in company.
    i wish i must have seen this questions before my test.
    Just 4 hours before.
    But yes this is life we all come to know afterwards.
    But the questions are really interesting.
    and from now onwards i will refer this site most of the time i can.
    and i will tell u my result of this test.

    Like

  30. Hi

    I am arun pandey working in MNCS . i have read ur tutorials & want to say really its very very good for preparing on sql server.

    i am very much interested in sql server stored procedure so plz give some extra on SP.

    Thanks
    Arun

    Like

  31. Hi
    Ur questions are great here are some more questions

    • What is normalization? Explain different levels of normalization?
    o 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.
    • What is denormalization and when would you go for it?
    o 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.
    • How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables?
    o One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table. It will be a good idea to read up a database designing fundamentals text book.
    • What’s the difference between a primary key and a unique key?
    o 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.
    • What are user defined datatypes and when you should go for them?
    o User 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 tables. See sp_addtype, sp_droptype in books online.
    • What is bit datatype and what’s the information that can be stored inside a bit column?
    o 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.
    • Define candidate key, alternate key, composite key.
    o 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.
    • What are defaults? Is there a column to which a default can’t be bound?
    o 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 DEFAULT in books online.
    • What is a transaction and what are ACID properties?
    o 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. Explain different isolation levels 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. Read Committed – A transaction operating at the Read Committed level cannot see changes made by other transactions until those transactions are committed. At this level of isolation, dirty reads are not possible but nonrepeatable reads and phantoms are possible. Read Uncommitted – A transaction operating at the Read Uncommitted level can see uncommitted changes made by other transactions. At this level of isolation, dirty reads, nonrepeatable reads, and phantoms are all possible. Repeatable Read – A transaction operating at the Repeatable Read level is guaranteed not to see any changes made by other transactions in values it has already read. At this level of isolation, dirty reads and nonrepeatable reads are not possible but phantoms are possible. Serializable – A transaction operating at the Serializable level guarantees that all concurrent transactions interact only in ways that produce the same effect as if each transaction were entirely executed one after the other. At this isolation level, dirty reads, nonrepeatable reads, and phantoms are not possible.
    • CREATE INDEX myIndex ON myTable(myColumn)What type of Index will get created after executing the above statement?
    o Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.
    • What’s the maximum size of a row?
    o 8060 bytes. Don’t be surprised with questions like ‘what is the maximum number of columns per table’. 1024 columns per table. Check out SQL Server books online 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 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. 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.
    • What is lock escalation?
    o 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.
    • What’s the difference between DELETE TABLE and TRUNCATE TABLE commands?
    o 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. TRUNCATE TABLE is functionally identical to DELETE statement with no WHERE clause: both remove all rows in the table. But TRUNCATE TABLE is faster and uses fewer system and transaction log resources than DELETE. The DELETE statement removes rows one at a time and records an entry in the transaction log for each deleted row. TRUNCATE TABLE removes the data by deallocating the data pages used to store the table’s data, and only the page deallocations are recorded in the transaction log. TRUNCATE TABLE removes all rows from a table, but the table structure and its columns, constraints, indexes and so on remain. The counter used by an identity for new rows is reset to the seed for the column. If you want to retain the identity counter, use DELETE instead. If you want to remove table definition and its data, use the DROP TABLE statement. You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint; instead, use DELETE statement without a WHERE clause. Because TRUNCATE TABLE is not logged, it cannot activate a trigger. TRUNCATE TABLE may not be used on tables participating in an indexed view
    • Explain the storage models of OLAP
    o Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more infomation.
    • What are the new features introduced in SQL Server 2000 (or the latest release of SQL Server at the time of your interview)? What changed between the previous version of SQL Server and the current version?
    o 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.
    • What are constraints? Explain different types of constraints.
    o Constraints enable the RDBMS enforce the integrity of the database automatically, 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 pages titled: “Constraints” and “CREATE TABLE”, “ALTER TABLE”
    • What is an index? What are the types of indexes? How many clustered indexes can be created on a table? I create a separate index on each column of a table. What are the advantages and disadvantages of this approach?
    o Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quicker. Indexes are of two types. Clustered indexes and non-clustered indexes. When you create a clustered index on a table, all the rows in the table are stored in the order of the clustered index key. So, there can be only one clustered index per table. Non-clustered indexes have their own storage separate from the table data storage. Non-clustered indexes 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, depending up on the absence or presence of clustered index on the table. If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same t ime, data modification operations (such as INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used.
    • What is RAID and what are different types of RAID configurations?
    o 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
    • What are the steps you will take to improve performance of a poor performing query?
    o 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 tables. Some of the tools/ways that help you troubleshooting performance problems are: SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SET STATISTICS IO ON, SQL Server Profiler, Windows NT /2000 Performance monitor, Graphical execution plan in Query Analyzer. Download the white paper on performance tuning SQL Server from Microsoft 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 an SQL Server?
    o 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. 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 about resolving deadlocks?
    o 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. Also check out the article Q169960 from Microsoft knowledge base.
    • What is blocking and how would you troubleshoot it?
    o 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%?
    o 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?
    o 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?
    o 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?
    o 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?
    o 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?
    o 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?
    o 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?
    o 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.
    • What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?
    o Cursors allow row-by-row processing of the resultsets. Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books 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 query makes only one roundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Further, there are restrictions on the SELECT statements that can be used with some types of cursors. Most of the times, set based operations can be used instead of cursors. Here is an example: If you have to give a flat hike to your employees using the following criteria: Salary between 30000 and 40000 — 5000 hike Salary between 40000 and 55000 — 7000 hike Salary between 55000 and 65000 — 9000 hike. In this situation many developers tend to use a cursor, determine each employee’s salary and update his salary according to the above formula. But the same can be achieved by multiple update statements or can be combined in a single UPDATE statement as shown below:
    o UPDATE tbl_emp SET salary = CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000 WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000 WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000 END
    o Another situation in which developers tend to use cursors: You need to call a stored procedure when a column in a particular row meets certain condition. You don’t have to use cursors for this. This can be achieved using WHILE loop, as long as there is a unique key to identify each row. For examples of using WHILE loop for row by row processing, check out the ‘My code library’ section of my site or search for WHILE. Write down the general syntax for a SELECT statements covering all the options. Here’s the basic syntax: (Also checkout SELECT in books online for advanced syntax).
    o 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.
    o Joins are 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. Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS. For more information see pages from books online titled: “Join Fundamentals” and “Using Joins”.
    • Can you have a nested transaction?
    o Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
    • What is an extended stored procedure? Can you instantiate a COM object by using T-SQL?
    o An 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 Server. Yes, you can instantiate a COM (written in languages like VB, VC++) object from T-SQL by using sp_OACreate stored procedure. Also see books online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty, sp_OADestroy. For an example of creating a COM object in VB and calling 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?
    o 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 to invoke a trigger on demand?
    o Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table. In SQL Server 6.5 you could define only 3 triggers per table, one for INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you could create multiple triggers per each action. But in 7.0 there’s no way to control the order in which the triggers fire. In SQL Server 2000 you could specify which trigger fires first or fires last using sp_settriggerorder. Triggers can’t be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which 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 of triggers, as constraints are much faster. Till SQL Server 7.0, triggers fire only after the data modification operation happens. So in a way, they are called post triggers. But in SQL Server 2000 you could create pre triggers also. Search SQL Server 2000 books online for INSTEAD OF triggers. Also check out books online for ‘inserted table’, ‘deleted table’ and COLUMNS_UPDATED()
    • There is a trigger defined for INSERT operations on a table, in an OLTP system. The trigger is written to instantiate a COM object and pass the newly insterted rows to it for some custom processing. What do you think of this implementation? Can this be implemented better?
    o 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.
    • What is a self join? Explain it with an example.
    o Self join 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.
    o CREATE TABLE emp ( empid int, mgrid int, empname char(10) )
    o 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’
    o SELECT t1.empname [Employee], t2.empname [Manager] FROM emp t1, emp t2 WHERE t1.mgrid = t2.empid Here’s an advanced query using a LEFT OUTER JOIN that even returns the employees without managers (super bosses)
    o SELECT t1.empname [Employee], COALESCE(t2.empname, ‘No manager’) [Manager] FROM emp t1 LEFT OUTER JOIN emp t2 ON t1.mgrid = t2.empid

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  32. hi,

    I need a help.

    we have a scenerio in which multiple users inserts around 15000+ records from xml file to sql2005 and my application has to update an id for each of these records according to users.

    During these process no other transactions should not work on same table (which will result in data corruption).

    I need to know whether sql automatically executes tablock or should I implement it myself inside storedprocedure.

    It will be very useful for me and my team if you can give us a suggestion.

    Thanks & Regds
    Hareesh

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  33. Hi Pinal,

    I want to write sp for following.

    To find out the result of no of items issued more than (5 or else) times with the input of from date and to date

    it will have more number of items.within that how to find out no of items issued more than 5 times………

    Rajeev

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  34. Hi,

    Good work. I like the way you laid out the questions and answered them in a simple manner.

    One suggestion:

    This relates to sql server 2000.
    can you add more information related to sql server 2005?
    Thanks in advance,
    Madhu

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  35. hello sir………
    can i get some common quetions asked on oracle during intervieve time………….

    Like

  36. Dear Sir,

    How i can find total tables and no of count records for each table in a single database i am using SQLserver

    plz help me

    thanks
    Krishna

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  37. Very good keep it up i love Indians ! i m from Pak but i like the way that you help and supports people no matter where they belong ………..

    Like

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