These material are compiled for helping junior / senior software engineers and
1. The most important DDL statements in SQL are:
CREATE TABLE - creates a new database table
ALTER TABLE - alters (changes) a database table
DROP TABLE - deletes a database table
CREATE INDEX - creates an index (search key)
DROP INDEX - deletes an index
2. Operators used in SELECT statements.
<> or != Not equal
> Greater than
< Less than
>= Greater than or equal
<= Less than or equal
BETWEEN Between an inclusive range
LIKE Search for a pattern
3. SELECT statements:
SELECT column_name(s) FROM table_name
SELECT DISTINCT column_name(s) FROM table_name
SELECT column FROM table WHERE column operator value
SELECT column FROM table WHERE column LIKE pattern
SELECT column,SUM(column) FROM table GROUP BY column
SELECT column,SUM(column) FROM table GROUP BY column HAVING SUM(column)
Note that single quotes around text values and numeric values should not be
enclosed in quotes. Double quotes may be acceptable in some databases.
4. The SELECT INTO Statement is most often used to create backup
copies of tables or for archiving records.
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source
SELECT column_name(s) INTO newtable [IN externaldatabase] FROM source WHERE
column_name operator value
5. The INSERT INTO Statements:
INSERT INTO table_name VALUES (value1, value2,....)
INSERT INTO table_name (column1, column2,...) VALUES (value1, value2,....)
6. The Update Statement:
UPDATE table_name SET column_name = new_value WHERE column_name =
7. The Delete Statements:
DELETE FROM table_name WHERE column_name = some_value
Delete All Rows:
DELETE FROM table_name or DELETE * FROM table_name
8. Sort the Rows:
SELECT column1, column2, ... FROM table_name ORDER BY columnX, columnY,
SELECT column1, column2, ... FROM table_name ORDER BY columnX DESC
SELECT column1, column2, ... FROM table_name ORDER BY columnX DESC, columnY
9. The IN operator may be used if you know the exact value you
want to return for at least one of the columns.
SELECT column_name FROM table_name WHERE column_name IN (value1,value2,..)
10. BETWEEN ... AND
SELECT column_name FROM table_name WHERE column_name BETWEEN value1 AND
value2 The values can be numbers, text, or dates.
11. What is the use of CASCADE CONSTRAINTS?
When this clause is used with the DROP command, a parent table can be
dropped even when a child table exists.
12. Why does the following command give a compilation
DROP TABLE &TABLE_NAME; Variable names should start with an alphabet. Here
the table name starts with an '&' symbol.
13. Which system tables contain information on privileges granted
and privileges obtained?
14. Which system table contains information on constraints on all
the tables created?obtained?
15. What is the difference between TRUNCATE and DELETE
16. State true or false. !=, <>, ^= all denote the same
17. State true or false. EXISTS, SOME, ANY are operators in
18. What will be the output of the following query?
SELECT REPLACE(TRANSLATE(LTRIM(RTRIM('!! ATHEN !!','!'), '!'), 'AN',
'**'),'*','TROUBLE') FROM DUAL;?
19. What does the following query do?
SELECT SAL + NVL(COMM,0) FROM EMP;?
This displays the total salary of all employees. The null values in the
commission column will be replaced by 0 and added to salary.
20. What is the advantage of specifying WITH GRANT OPTION in the
The privilege receiver can further grant the privileges he/she has
obtained from the owner to any other user.
21. Which command executes the contents of a specified
START or @.
22. What is the value of comm and sal after executing the
following query if the initial value of sal is 10000
UPDATE EMP SET SAL = SAL + 1000, COMM = SAL*0.1;?
sal = 11000, comm = 1000.
23. Which command displays the SQL command in the SQL buffer, and
then executes it?
24. What command is used to get back the privileges offered by the
25. What will be the output of the following query? SELECT
DECODE(TRANSLATE('A','1234567890','1111111111'), '1','YES', 'NO' );?
Explanation : The query checks whether a given string is a numerical
26. Which date function is used to find the difference between two
27. What operator performs pattern matching?
28. What is the use of the DROP option in the ALTER TABLE
It is used to drop constraints specified on the table.
29. What operator tests column for the absence of data?
IS NULL operator.
30. What are the privileges that can be granted on a table by a
user to others?
Insert, update, delete, select, references, index, execute, alter, all.
31. Which function is used to find the largest integer less than
or equal to a specific value?
32. Which is the subset of SQL commands used to manipulate Oracle
Database structures, including tables?
Data Definition Language (DDL).
33. What is the use of DESC in SQL?
DESC has two purposes. It is used to describe a schema as well as to
retrieve rows from table in descending order.
The query SELECT * FROM EMP ORDER BY ENAME DESC will display the output
sorted on ENAME in descending order.
34. What command is used to create a table by copying the
structure of another table?
CREATE TABLE .. AS SELECT command
To copy only the structure, the WHERE clause of the SELECT command should
contain a FALSE statement as in the following.
CREATE TABLE NEWTABLE AS SELECT * FROM EXISTINGTABLE WHERE 1=2;
If the WHERE condition is true, then all the rows or rows satisfying the
condition will be copied to the new table.
35. TRUNCATE TABLE EMP;
DELETE FROM EMP;
Will the outputs of the above two commands differ?
Both will result in deleting all the rows in the table EMP..
36. What is the output of the following query SELECT
TRUNC(1234.5678,-2) FROM DUAL;?
37. What are the wildcards used for pattern matching.?
_ for single character substitution and % for multi-character
38. What is the parameter substitution symbol used with INSERT
39. What's an SQL injection?
SQL Injection is when form data contains an SQL escape sequence and
injects a new SQL query to be run.
- How do you implement one-to-one, one-to-many and many-to-many
relationships while designing tables?
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
It will be a good idea to read up a database designing fundamentals text
- What's the difference between a primary key and a unique
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 nique key
allows one NULL only.
- What are user defined datatypes and when you should go for
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 is bit datatype and what's the information that can be
stored inside a bit column?
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
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
- What are defaults? Is there a column to which a default can't
A default is a value that will be used by a column, if no value is supplied
to that column while inserting data. IDENTITY columns and timestamp columns
can't have defaults bound to them. See CREATE DEFUALT in books online.
- What is a transaction and what are ACID
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 efault SQL Server solation 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.
- CREATE INDEX myIndex ON myTable(myColumn)
What type of Index will get created after executing the above
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?
8060 bytes. Don't be surprised with questions like 'what is the maximum
number of columns per table'. Check out SQL Server books online for the
page titled: "Maximum Capacity Specifications".
- Explain Active/Active and Active/Passive cluster
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
- 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 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
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.
- Explain the storage models of OLAP
Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more
- 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
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
- What are constraints? Explain different types of
Constraints enable the RDBMS enforce the integrity of the database
automatically, without needing you to create triggers, rule or
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"
- Whar 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
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 is RAID and what are different types of RAID
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?
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?
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.
- What is a deadlock and what is a live lock? How will you go
about resolving deadlocks?
Deadlock is a situation when two processes, each having a lock on one piece
of data, attempt to acquire a lock on the other's piece. Each process
would wait indefinitely for the other to release the lock, unless one of
the user processes is terminated. SQL Server detects deadlocks and
terminates one user's process.
A livelock is one, where a request for an exclusive lock is repeatedly
denied because a series of overlapping shared locks keeps interfering. SQL
Server detects the situation after four denials and refuses further shared
locks. A livelock also occurs when read transactions monopolize a table or
page, forcing a write transaction to wait indefinitely.
Check out SET DEADLOCK_PRIORITY and "Minimizing Deadlocks" in SQL Server
books online. Also check out the article Q169960 from Microsoft knowledge
- What is blocking and how would you troubleshoot
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 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
- How to restart SQL Server in single user mode? How to start SQL
Server in minimal configuration mode?
SQL Server can be started from command line, using the SQLSERVR.EXE. This
EXE has some very important parameters with which a DBA should be familiar
with. -m is used for starting SQL Server in single user mode and -f is used
to start the SQL Server in minimal confuguration mode. Check out SQL Server
books online for more parameters and their explanations.
- As a part of your job, what are the DBCC commands that you
commonly use for database maintenance?
DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC, DBCC
SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there are a whole
load of DBCC commands which are very useful for DBAs. Check out SQL Server
books online for more information.
- What are statistics, under what circumstances they go out of
date, how do you update them?
Statistics determine the selectivity of the indexes. If an indexed column
has unique values then the selectivity of that index is more, as opposed to
an index with non-unique values. Query optimizer uses these indexes in
determining whether to choose an index or not while executing a query.
Some situations under which you should update statistics:
1) If there is significant change in the key values in the index
2) If a large amount of data in an indexed column has been added, changed,
or removed (that is, if the distribution of key values has changed), or the
table has been truncated using the TRUNCATE TABLE statement and then
3) Database is upgraded from a previous version
Look up SQL Server books online for the following commands: UPDATE
STATISTICS, STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS, DROP
STATISTICS, sp_autostats, sp_createstats, sp_updatestats
- What are the different ways of moving data/databases between
servers and databases in SQL Server?
There are lots of options available, you have to choose your option
depending upon your requirements. Some of the options you have are:
BACKUP/RESTORE, dettaching and attaching databases, replication, DTS, BCP,
logshipping, INSERT...SELECT, SELECT...INTO, creating INSERT scripts to
- Explian different types of BACKUPs avaialabe in SQL Server?
Given a particular scenario, how would you go about choosing a backup
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 replicaion? What are the different types of
replication you can set up in SQL Server?
Replication is the process of copying/moving data between databases on the
same or different servers. SQL Server supports the following types of
- 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
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
- What are cursors? Explain different types of cursors. What
are the disadvantages of cursors? How can you avoid
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
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
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
- 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
[GROUP BY group_by_expression]
[ORDER BY order_expression [ASC | DESC] ]
- What is a join and explain different types of
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
Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs.OUTER JOINs are
further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL
For more information see pages from books online titled: "Join
Fundamentals" and "Using Joins".
- Can you have a nested transaction?
Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and
- What is an extended stored procedure? Can you instantiate a
COM object by using T-SQL?
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
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,
- What is the system function to get the current user's user
USER_ID().Also check out other system functions like USER_NAME(),
SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(),
- What are triggers? How many triggers you can have on a
table? How to invoke a trigger on demand?
Triggers are special kind of stored procedures that get executed
automatically when an INSERT, UPDATE or DELETE operation takes place on
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
- 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
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
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
CREATE TABLE emp
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')
FROM emp t1
LEFT OUTER JOIN
t1.mgrid = t2.empid
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