Sunday 28 April 2013

SQL Interview Questions and Answers



SQL Interview Questions and Answers :


What is the difference between oracle,sql and sql server ?

Oracle is based on RDBMS.
SQL is Structured Query Language.
SQL Server is another tool for RDBMS provided by MicroSoft.
why you need indexing ? where that is stroed and what you mean by schema object? For what purpose we are using view?

We cant create an Index on Index.. Index is stoed in user_index table.Every object that has been created on Schema is Schema Object like Table,View etc.If we want to share the particular data to various users we have to use the virtual table for the Base table...So tht is a view.

indexing is used for faster search or to retrieve data faster from various table. Schema containing set of tables, basically schema means logical separation of the database. View is crated for faster retrieval of data. It's customized virtual table. we can create a single view of multiple tables. Only the drawback is..view needs to be get refreshed for retrieving updated data.

Difference between Store Procedure and Trigger?

we can call stored procedure explicitly.
but trigger is automatically invoked when the action defined in trigger is done.
ex: create trigger after Insert on
this trigger invoked after we insert something on that table.
Stored procedure can't be inactive but trigger can be Inactive.
Triggers are used to initiate a particular activity after fulfilling certain condition.It need to define and can be enable and disable according to need.


What is the advantage to use trigger in your PL?

Triggers are fired implicitly on the tables/views on which they are created. There are various advantages of using a trigger. Some of them are:

Suppose we need to validate a DML statement(insert/Update/Delete) that modifies a table then we can write a trigger on the table that gets fired implicitly whenever DML statement is executed on that table.
Another reason of using triggers can be for automatic updation of one or more tables whenever a DML/DDL statement is executed for the table on which the trigger is created.
Triggers can be used to enforce constraints. For eg : Any insert/update/ Delete statements should not be allowed on a particular table after office hours. For enforcing this constraint Triggers should be used.
Triggers can be used to publish information about database events to subscribers. Database event can be a system event like Database startup or shutdown or it can be a user even like User loggin in or user logoff.
What the difference between UNION and UNIONALL?

Union will remove the duplicate rows from the result set while Union all does'nt.

What is the difference between TRUNCATE and DELETE commands?

Both will result in deleting all the rows in the table .TRUNCATE call cannot be rolled back as it is a DDL command and all memory space for that table is released back to the server. TRUNCATE is much faster.Whereas DELETE call is an DML command and can be rolled back.

Which system table contains information on constraints on all the tables created ?
yes,
USER_CONSTRAINTS,
system table contains information on constraints on all the tables created

Explain normalization ?
Normalisation means refining the redundancy and maintain stablisation. there are four types of normalisation :
first normal forms, second normal forms, third normal forms and fourth Normal forms.

How to find out the database name from SQL*PLUS command prompt?
Select * from global_name;
This will give the datbase name which u r currently connected to.....

What is the difference between SQL and SQL Server ?

SQLServer is an RDBMS just like oracle,DB2 from Microsoft
whereas
Structured Query Language (SQL), pronounced "sequel", is a language that provides an interface to relational database systems. It was developed by IBM in the 1970s for use in System R. SQL is a de facto standard, as well as an ISO and ANSI standard. SQL is used to perform various operations on RDBMS.

What is diffrence between Co-related sub query and nested sub query?

Correlated subquery runs once for each row selected by the outer query. It contains a reference to a value from the row selected by the outer query.

Nested subquery runs only once for the entire nesting (outer) query. It does not contain any reference to the outer query row.

For example,

Correlated Subquery:

select e1.empname, e1.basicsal, e1.deptno from emp e1 where e1.basicsal = (select max(basicsal) from emp e2 where e2.deptno = e1.deptno)

Nested Subquery:

select empname, basicsal, deptno from emp where (deptno, basicsal) in (select deptno, max(basicsal) from emp group by deptno)

WHAT OPERATOR PERFORMS PATTERN MATCHING?
Pattern matching operator is LIKE and it has to used with two attributes

1. % and

2. _ ( underscore )

% means matches zero or more characters and under score means mathing exactly one character

1)What is difference between Oracle and MS Access?
2) What are disadvantages in Oracle and MS Access?
3) What are feratures&advantages in Oracle and MS Access?
Oracle's features for distributed transactions, materialized views and replication are not available with MS Access. These features enable Oracle to efficiently store data for multinational companies across the globe. Also these features increase scalability of applications based on Oracle.

What is database?
A database is a collection of data that is organized so that itscontents can easily be accessed, managed and updated. open this url : http://www.webopedia.com/TERM/d/database.html

What is cluster.cluster index and non cluster index ?
Clustered Index:- A Clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table may have only one clustered index.Non-Clustered Index:- A Non-Clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows in the disk. The leaf nodes of a non-clustered index does not consists of the data pages. instead the leaf node contains index rows.

How can i hide a particular table name of our schema?
you can hide the table name by creating synonyms.

e.g) you can create a synonym y for table x

create synonym y for x;

What is difference between DBMS and RDBMS?
The main difference of DBMS & RDBMS is

RDBMS have Normalization. Normalization means to refining the redundant and maintain the stablization.
the DBMS hasn't normalization concept.

What are the advantages and disadvantages of primary key and foreign key in SQL?

Primary key

Advantages

1) It is a unique key on which all the other candidate keys are functionally dependent

Disadvantage

1) There can be more than one keys on which all the other attributes are dependent on.

Foreign Key

Advantage

1)It allows refrencing another table using the primary key for the other table

Which date function is used to find the difference between two dates?
datediff

for Eg: select datediff (dd,'2-06-2007','7-06-2007')

output is 5


What is denormalization and when would you go for it?


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?


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?


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?


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?


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.


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?


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.


Back to top
SQL Server architecture       (top)


What is a transaction and what are ACID properties?


A transaction is a logical unit of work in which, all the steps 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.


CREATE INDEX myIndex ON myTable(myColumn)


What type of Index will get created after executing the above statement?


Non-clustered index. Important thing to note: By default a 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 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?


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?


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
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?


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.


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"


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
approach?


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.


Back to top
Database administration       (top)


What is RAID and what are different types of RAID configurations?


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.


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?


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?


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 information.


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 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?


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.


Explian different types of BACKUPs avaialabe in SQL Server? Given a
particular scenario, how would you go about choosing a backup plan?


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 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?


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.


Back to top
Database programming       (top)


What are cursors? Explain different types of cursors. What are the
disadvantages of cursors? How can you avoid cursors?


Cursors allow row-by-row prcessing of the resultsets.


Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. 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. 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).


SELECT select_list
[INTO new_table_]
FROM table_source
[WHERE search_condition]
[GROUP BY group_by__expression]
[HAVING search_condition]
[ORDER BY order__expression [ASC | DESC] ]


What is a join and explain different types of joins.


Joins are used in queries to explain how different tables are related.
Joins also let you select data from a table depending upon data 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?


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?


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?


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?


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?


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.


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


What is normalization? Explain different levels of normalization?


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.
Frequently Asked Questions about MS-SQl server :


Why is a UNION ALL faster than a UNION?

UNION ALL faster than a UNION because for union operation server needs to remove the duplicate values but for union all its not. Thats why the UNOIN ALL is fater than UNION Operation. It is recommended that if you know that the union set operation never returns duplicate values than you must use UNION ALL instead of UNION.

How many types of data models are there?

There are no standards in this area. Authors and theorists make it up as they go. The entity-relationship model (ER) has hundreds of derivitives (bachman, chen, ibm, IDEF1x etc.). the most popular of the OO models is Unified Modeling Language (UML). Actually UML and IDEF1x are closest to becoming a standard that can support software products. Rational already has products and IDEF1x is the language of ERwin.
Don't be fooled by these variations. They all represent the same things, you have to be very careful that you understand all of the non-standard symbols or you will surely make mistakes in interpreting what the pictures mean.

What is denormalization and when would you go for it?

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.




What's the difference between a primary key and a unique key?

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.

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 are defaults? Is there a column to which a default can't be bound?
A default is a value that will be used by a column, if no value 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.

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 approach?

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 are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?

Cursors allow row-by-row prcessing of the resultsets.

Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. 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. For examples of using WHILE loop for row by row processing,

What is a join and explain different types of joins?

Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data 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.

What is a Stored Procedure?

Its nothing but a set of T-SQL statements combined to perform a single task of several tasks. Its basically like a Macro so when you invoke the Stored procedure, you actually run a set of statements.

What is the basic difference between clustered and a non-clustered index?

The difference is that, Clustered index is unique for any given table and we can have only one clustered index on a table. The leaf level of a clustered index is the actual data and the data is resorted in case of clustered index. Whereas in case of non-clustered index the leaf level is actually a pointer to the data in rows so we can have as many non-clustered indexes as we can on the db.

What are cursors?

Well cursors help us to do an operation on a set of data that we retreive by commands such as Select columns from table. For example : If we have duplicate records in a table we can remove it by declaring a cursor which would check the records during retreival one by one and remove rows which have duplicate values.

Which TCP/IP port does SQL Server run on?

SQL Server runs on port 1433 but we can also change it for better security.

Can we use Truncate command on a table which is referenced by FOREIGN KEY?

No. We cannot use Truncate command on a table with Foreign Key because of referential integrity.

What is the use of DBCC commands?

DBCC stands for database consistency checker. We use these commands to check the consistency of the databases, i.e., maintenance, validation task and status checks.

What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?

Having Clause is basically used only with the GROUP BY function in a query. WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.

What is a Linked Server?

Linked Servers is a concept in SQL Server by which we can add other SQL Server to a Group and query both the SQL Server dbs using T-SQL Statements.

Can you link only other SQL Servers or any database servers such as Oracle?

We can link any server provided we have the OLE-DB provider from Microsoft to allow a link. For Oracle we have a OLE-DB provider for oracle that microsoft provides to add it as a linked server to the sql server group.

How do you troubleshoot SQL Server if its running very slow?

First check the processor and memory usage to see that processor is not above 80% utilization and memory not above 40-45% utilization then check the disk utilization using Performance Monitor, Secondly, use SQL Profiler to check for the users and current SQL activities and jobs running which might be a problem. Third would be to run UPDATE_STATISTICS command to update the indexes.

What is log shipping?

Can we do logshipping with SQL Server 7.0 - Logshipping is a new feature of SQL Server 2000. We should have two SQL Server - Enterprise Editions. From Enterprise Manager we can configure the logshipping. In logshipping the transactional log file from one server is automatically updated into the backup database on the other server. If one server fails, the other server will have the same db and we can use this as the DR (disaster recovery) plan.

Let us say the SQL Server crashed and you are rebuilding the databases including the master database what procedure to you follow?

For restoring the master db we have to stop the SQL Server first and then from command line we can type SQLSERVER .m which will basically bring it into the maintenance mode after which we can restore the master db.

What is BCP? When do we use it?

BulkCopy is a tool used to copy huge amount of data from tables and views. But it won’t copy the structures of the same.


Saturday 27 April 2013

LoadRunner Tutorial



3
Building Scripts
To create load, you first build automated scripts that emulate real user
behavior.
In this lesson you will cover the following topics:
Introducing the Virtual User Generator (VuGen)
How do I start recording user activities?
Using VuGen Wizard Mode
How do I record a business process to create a script?
How do I view the script?
Introducing the Virtual User Generator (VuGen)
In a testing environment, LoadRunner replaces human users at physical
machines, with virtual users, or a Vusers. Vusers create load on a system by
emulating actions of typical users in a repeatable and predictable manner.
The LoadRunner Virtual User Generator (VuGen) works on a record-andplayback
principle. As you walk through a business process on your
application, VuGen records your actions into automated scripts which will
form the foundation of your load tests.
Note: If you have completed the HP LoadRunner Quick Start, you will
notice that the script steps that you recorded there are identical to those
that you will record in the following section. However, the overall recording
process is explained here in greater detail.

How do I start recording user activities?
To begin recording user actions, you open VuGen and create a blank script.
You fill the blank script by recording events and adding manual
enhancements.
In this section, you will open VuGen and create a blank Web script.
1 Start LoadRunner.
Choose Start > Programs >LoadRunner>LoadRunner. The HP LoadRunner
Launcher window opens.
 

2 Open VuGen.
In the Launcher window, click the Load Testing tab.
Click Create/Edit Scripts. VuGen’s Start Page opens.

Using VuGen Wizard Mode
The empty script opens in VuGen’swizard mode with the Task pane
displayed on the left. (If the task pane is not displayed click the Tasks button
on the toolbar. If the Start Recording dialog box opens automatically, click
Cancel).
VuGen’s wizard takes you through a step by step process of creating a script
and then adapting it for your test environment.
The Task pane lists each step or task in the script creation process. As you
proceed through each step, VuGen displays detailed instructions and
guidelines in the main area of the window.
 
You can customize your VuGen window to show or hide the various
toolbars. To show or hide a toolbar, choose View > Toolbars and toggle the
check mark adjacent to the desired toolbar.
You return to the VuGen wizard at aany stage by opening the task pane and
clicking on one of the task steps.

How do I record a business process to create a script?
The next step in creating user emulation, is recording the events performed
by a real user. In the previous section, you created an empty Web script.
Now you can begin to record events directly into the script. In this section,
you will track the events of one passenger reserving a flight from Denver to
Los Angeles and then checking the flight itinerary.
To record the script:
1 Start recording on the HP Tours Web site.
Click Record Application in step 1 in the Task pane.
Click Start Recording at the bottom of the instruction pane.

Alternatively you can choose Vuser>Start Record or click the Start Record
button in the Toolbar at the top of the page.
9 Choose File > Save or click the Save button. Type basic_tutorialin the File
name box and click Save. VuGen saves the file in the LoadRunner script
folder and displays the test name in the title bar.
How do I view the script?
You have now recorded a travel agent logging in, booking a flight, and
logging off. VuGen recorded your steps from the moment you clicked the
Start Record button to the moment you clicked the Stop button.
You can now view the script inside VuGen. You can view the script in Tree
viewor Script view. Tree view is an icon-based view that lists the actions of
theVuser as steps, while Script view is a text-based view that lists the actions
of the Vuser as functions.



Tree View
To view the script in Tree view choose View > Tree View or click the Tree
view button. To view the Tree View across the whole window, remove the
Task pane by clicking the Task button.
 
For each step you performed during recording, VuGen generated an icon
and a title in the test tree. In Tree view, you see the actions of the user as
script steps. Most steps are accompanied by a corresponding snapshot of the
recording.
The snapshots make the scripts easier to understand, and easier to share
between engineers because you can see exactly which screen was recorded
during the recording process. You can compare the snapshots afterwards to
verify your script’s accuracy. VuGen also creates snapshots of each step
during replay.
Click the plus (+) sign adjacent to any of the steps in the test tree. You now
see the Think Time that was recorded while you were booking a flight. Think
Time represents the actual time you waited between steps, and can be used
to emulate fast and slow user behavior under load. Think time is a
mechanism whereby you can make your load test more accurately reflect a
real user’s behavior.

Script View
Script view is a text-based view that lists the actions of the Vuser as API
functions. To view the script in Script view choose View > Script View or
click the Script View button.

In Script view, VuGen shows the script in an editor with color coded
functions and their argument values. You can type C or LoadRunner API
functions, as well as control flow statements, directly into this window