A.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.
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 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)
SELECT t1.empname [Employee], COALESCE(t2.empname, 'No manager') [Manager]
FROM emp t1
LEFT OUTER JOIN
emp t2
ON
t1.mgrid = t2.empid
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?
A.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 are triggers? How many triggers you can have on a table? How to invoke a trigger on demand?
A.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()
What is the system function to get the current user's user id?
A.USER_ID(). Also check out other system functions like USER_NAME(), SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().
What is an extended stored procedure? Can you instantiate a COM object by using T-SQL?
A.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.
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.
Can you have a nested transaction?
A.Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
What is a join and explain different types of joins.
A.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.
Write down the general syntax for a SELECT statements covering all the options.
A.Here's the basic 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 are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?
A.Cursors allow row-by-row prcessing 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 rowundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Furthere, 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:
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
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.
What is database replicaion? What are the different types of replication you can set up in SQL Server?
A.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
A.Types of backups you can create in SQL Sever 7.0+ are Full database backup, differential database backup, transaction log backup, filegroup backup.
What are the different ways of moving data/databases between servers and databases in SQL Server?
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.
What are statistics, under what circumstances they go out of date, how do you update them
A.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
As a part of your job, what are the DBCC commands that you commonly use for database maintenance?
A.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
How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode?
A.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.
Explain CREATE DATABASE syntax
A.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.
What is blocking and how would you troubleshoot it?
A.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.
What is a deadlock and what is a live lock? How will you go about resolving deadlocks?
A.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.
What are the steps you will take, if you are tasked with securing an SQL Server?
A.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.
http://vyaskn.tripod.com/sql_server_security_best_practices.htm
What are the steps you will take to improve performance of a poor performing query?
A.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.
What is RAID and what are different types of RAID configurations?
A.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.
http://www.raid-advisory.com/
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?
A.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 craete 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's the difference between DELETE TABLE and TRUNCATE TABLE commands?
A.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.
What is lock escalation?
A.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.
CREATE INDEX myIndex ON myTable(myColumn)
What type of Index will get created after executing the above statement?
A.Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.
Explain different isolation levels
A.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.
What is a transaction and what are ACID properties?
A.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.
What are defaults? Is there a column to which a default can't be bound?
A.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.
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.
What is bit datatype and what's the information that can be stored inside a bit column?
A.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.
What are user defined datatypes and when you should go for them?
A.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.
What's the difference between a primary key and a unique key?
A.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.
How you can rename a database ?
A. To rename a database first get the database into Single user mode . by using the query below ( run these query in query analyzer)
ALTER DATABASE DBMydb SET SINGLE_USER WITH ROLLBACK IMMEDIATE
then rename the database using sp_rename
sp_renamedb 'DBMydb', 'DBmydb_new'
then change the access mode of the database to multiuser
ALTER DATABASE DBmydb_new SET MULTI_USER
What is mean by suspect status of a database ?
A. SQL Server gives a database 'suspect' status when it is unable to recover the database . reasons may be many , may be a torn page detection , may be datafile location no found.
How many access mode of database are in MSSQL
A there are three access mode of a database SINGLE_USER , RESTRICTED_USER and MULTI_USER . SINGLE_USER mode restricts database access to one user Connection. RESTRICTED_USER mode restricts database access to db_owner, dbcreator, and sysadmin roles. All other users are disconnected.MULTI_USER mode lets all users with proper access connect to the database.
What are the default databases in MSSQL Server Instance after Fresh installation
A. there are 4 default databases .
- Master
This is your most critical database, and is the core of your SQL Server implementation.
Limit changes and access to this database as much as possible. Without the master
database, your SQL Server instance will not start up. - MSDB
This database contains tables that control and track the elements of the SQL Server
Agent, as well as replication, log shipping (Enterprise Edition), database backup and
restore history, database maintenance plan configurations and history, and a subset of
tables participating in replication status and jobs. DTS packages saved to SQL Server are also stored within this database. - Model
The model database is a template database, which, when modified, will affect the default
values of databases that are created in the future. For example, if a table is added to the
model database, future databases created for the SQL Server instance will include this
table. - Tempdb
The tempdb database is used to hold all temporary tables and temporary stored
procedures. SQL Server also uses it for internal sorting and query operations (called
work tables, or spooling). This database is re-created each time SQL Server is started,
and cannot be used to store permanent tables.
What is replication ?
A. Replication is a process which enables data and database objects to be copied and modified from one database to another across different networks and platforms.the physical seperation of the databases and latency are integral part of the design process in replication. MSSQL Server 2000 permits 3 different kind of replication. they are snapshot,transactional and merge.
How many typf of backups are supported by MSSQL ?
A. there are three type of backup
- Full Database Backup.
- Transaction Log Backup
- Differential Backup
A. Log shipping is the process of backing up of database and transaction log files on a production SQL server, and then restoring them onto a standby server. The key feature of log shipping is that it will automatically backup transaction logs throughout the day (for whatever interval you specify) and automatically restore them on the standby server. This in effect keeps the two SQL Servers in "sync". Should the production server fail, all you have to do is point the users to the new server, and you are all set.
Name the types of Recovery model in SQL Server ?
A. There are thre recovery model in MSSQL Server
• Simple Recovery Model .
• Full Recovery Model.
• Bulk Logged Recovery Model.
What is a Dead Lock ?
A . Deadlock occurs when two users have locks on separate objects and each user wants a lock on the other's object. For example, User1 has a lock on object "A" and wants a lock on object "B" and User2 has a lock on object "B" and wants a lock on object "A". In this case, SQL Server ends a deadlock by choosing the user, who will be a deadlock victim. After that, SQL Server rolls back the breaking user's transaction, sends message number 1205 to notify the user's application about breaking, and then allows the nonbreaking user's process to continue.
Note. To reduce the chance of a deadlock, you should minimize the size of transactions and transaction times.
How many type of Locks are there ?
A. There are three main types of locks that SQL Server 7.0/2000 uses
Shared locks
Update locks
Exclusive locks
A database is consists of how many type of files ? Name their extensions
A. A database can consist of three types of file :
Primary file , Seconday file and transaction logs
The primary data file is the starting point of the database and points to any other files in the database. If the primary file is not large enough to hold all the data in the database, then secondary files are required.Transaction logs files hold records of updates to the data files, and can be used to recover the database after an error. Every database has at least one transaction log file, and some databases may have more than one.
The recommended file name extension for primary data files is .mdf.
The recommended file name extension for secondary data files is .ndf.
The recommended file name extension for log files is .ldf
How many type of indexes are there ? How many Index can be created on a table ?
A. there are two type of indexes Clustered and Non-Clustured, 249 Non Clustured and 1 Clustered index
How you can get the last identity value inserted in any table ?
A. SQL Server has a System Variable @@IDENTITY which gives the last identity element value inserted in any table
How many type of Contraints are in MSSQL.
A. SQL Server 2000 supports five classes of constraints.
1) NOT NULL
2) CHECK
3) UNIQUE
4) PRIMARY KEY
5) FOREIGN KEY
How you can trap error in Sql Server
A. by using @@error .
After each Transact-SQL statement, the server sets the variable to an integer value:
0—if the statement was successful
Error number—if the statement has failed
What is @@Fetch_status ?
A. @@fetch_status is a function (or global variable) that returns the success code of the last Fetch statement executed during the current connection. It is often used as an exit criterion in loops that fetch records from a cursor.
Whats the limitation of user defined funtion ?
A User-defined functions have one serious limitation. They cannot have side effects. A function side effect is any permanent change to resources (such as tables) that have a scope outside of the function (such as a non-temporary table that is not declared in the function). Basically, this requirement means that a function should return a value while changing nothing in the database. it means in short that "user defined function can not use UPDATE / DELETE on permament table objects in MSSQL"
In how many ways you can recieve information from stored procedure
A. there are 4 ways to receive information from a stored procedure:
Resultset , Parameters , Return value , A global cursor that can be referenced outside the stored procedure.
How many type of triggers are there in Sql Server.
A. There are two type of triggers
• After Triggers
• Instead of Triggers
Whats the Difference between datetime and smalldatetime datatype in SQL Server .
A . The main difference between these two datatypes is in the amount of space they occupy. datetime occupies eight bytes and smalldatetime only four. The difference in size is due to a difference in precision. The precision of smalldatetime is one minute, and it covers dates from January 1, 1900 , through June 6, 2079 , which is usually more than enough. The precision of datetime is 3.33 ms, and it covers dates from January 1, 1753 , to December 31, 9999 .
How many type of authentication method are there in SQL Server 2005 and 2000 and what is that ?
A. There are two type of authentication method in SQL Server 2005 and 2000
Windows Authentication
SQL Server Authentication
How can you generate GUID in in Transact-SQL ?
A. GUIDs can be generated using the NEWID function.
How many system datatypes are in SQL Server.
A. 27
What is the Maximum number of input and output parameters in Stored procedure in SQL Server 2000 ?
A. 1024
How can we improve stored procedure performance? Explain the performance tuning techniques.
1. Use many WHERE clauses in the SELECT statements.
2. Select only those fields which you really require.
3. Joins are expensive in terms of time. Join the tables using related fields manly indexed fields.
4. Don't use unused tables.
NOTES: When you execute the stored proc, SQL Server generates an execution plan based on your indexes and statistics available. Now it is possible that the stored proc inserts a lot of rows into some table which is used in another statement in a join and the plan that was built is not efficient for the subsequent sql statement because the profile of the table has changed since the plan was generated. I would suggest splitting the stored proc into two and the split should be done such that the second proc deals with the updated table. When the second proc is executed, it will have a more realistic execution plan for the updated table.
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