LOCK | SQL Server Interview Questions

1. What are locks?
   Microsoft® SQL Server™ 2000 uses locking to ensure transactional integrity and database consistency. Locking prevents users from reading data being changed by other users, and prevents multiple users from changing the same data at the same time. If locking is not used, data within the database may become logically incorrect, and queries executed against that data may produce unexpected results. 
2. What are the different types of locks?
   SQL Server uses these resource lock modes.

Lock mode	Description
Shared (S)	Used for operations that do not change or update data (read-only operations), such as a SELECT statement.
Update (U)	Used on resources that can be updated. Prevents a common form of deadlock that occurs when multiple sessions are reading, locking, and potentially updating resources later.
Exclusive (X)	Used for data-modification operations, such as INSERT, UPDATE, or DELETE. Ensures that multiple updates cannot be made to the same resource at the same time.
Intent	Used to establish a lock hierarchy. The types of intent locks are: intent shared (IS), intent exclusive (IX), and shared with intent exclusive (SIX).
Schema	Used when an operation dependent on the schema of a table is executing. The types of schema locks are: schema modification (Sch-M) and schema stability (Sch-S).
Bulk Update (BU)	Used when bulk-copying data into a table and the TABLOCK hint is specified.

3. What is a dead lock? Give a practical sample? How you can minimize the deadlock situation? 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.) 
4. What is isolation level?
   An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. A lower isolation level increases concurrency, but at the expense of data correctness. Conversely, a higher isolation level ensures that data is correct, but can affect concurrency negatively. The isolation level required by an application determines the locking behavior SQL Server uses.
   SQL-92 defines the following isolation levels, all of which are supported by SQL Server:
• Read uncommitted (the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read).
• Read committed (SQL Server default level).
• Repeatable read.
• Serializable (the highest level, where transactions are completely isolated from one another).

Isolation level	Dirty read	Nonrepeatable read	Phantom
Read uncommitted	Yes	Yes	Yes
Read committed	No	Yes	Yes
Repeatable read	No	No	Yes
Serializable	No	No	No

5. nolock? What is the difference between the REPEATABLE READ and SERIALIZE isolation levels?
   Locking Hints - A range of table-level locking hints can be specified using the SELECT, INSERT, UPDATE, and DELETE statements to direct Microsoft® SQL Server 2000 to the type of locks to be used. Table-level locking hints can be used when a finer control of the types of locks acquired on an object is required. These locking hints override the current transaction isolation level for the session.

Locking hint	Description
HOLDLOCK	Hold a shared lock until completion of the transaction instead of releasing the lock as soon as the required table, row, or data page is no longer required. HOLDLOCK is equivalent to SERIALIZABLE.
NOLOCK	Do not issue shared locks and do not honor exclusive locks. When this option is in effect, it is possible to read an uncommitted transaction or a set of pages that are rolled back in the middle of a read. Dirty reads are possible. Only applies to the SELECT statement.
PAGLOCK	Use page locks where a single table lock would usually be taken.
READCOMMITTED	Perform a scan with the same locking semantics as a transaction running at the READ COMMITTED isolation level. By default, SQL Server 2000 operates at this isolation level.
READPAST	Skip locked rows. This option causes a transaction to skip rows locked by other transactions that would ordinarily appear in the result set, rather than block the transaction waiting for the other transactions to release their locks on these rows. The READPAST lock hint applies only to transactions operating at READ COMMITTED isolation and will read only past row-level locks. Applies only to the SELECT statement.
READUNCOMMITTED	Equivalent to NOLOCK.
REPEATABLEREAD	Perform a scan with the same locking semantics as a transaction running at the REPEATABLE READ isolation level.
ROWLOCK	Use row-level locks instead of the coarser-grained page- and table-level locks.
SERIALIZABLE	Perform a scan with the same locking semantics as a transaction running at the SERIALIZABLE isolation level. Equivalent to HOLDLOCK.
TABLOCK	Use a table lock instead of the finer-grained row- or page-level locks. SQL Server holds this lock until the end of the statement. However, if you also specify HOLDLOCK, the lock is held until the end of the transaction.
TABLOCKX	Use an exclusive lock on a table. This lock prevents others from reading or updating the table and is held until the end of the statement or transaction.
UPDLOCK	Use update locks instead of shared locks while reading a table, and hold locks until the end of the statement or transaction. UPDLOCK has the advantage of allowing you to read data (without blocking other readers) and update it later with the assurance that the data has not changed since you last read it.
XLOCK	Use an exclusive lock that will be held until the end of the transaction on all data processed by the statement. This lock can be specified with either PAGLOCK or TABLOCK, in which case the exclusive lock applies to the appropriate level of granularity.

7. For example, if the transaction isolation level is set to SERIALIZABLE, and the table-level locking hint NOLOCK is used with the SELECT statement, key-range locks typically used to maintain serializable transactions are not taken.
   USE pubs
   GO
   SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
   GO
   BEGIN TRANSACTION
   SELECT au_lname FROM authors WITH (NOLOCK)
   GO

8. What is escalation of locks?
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.

TRIGGER | SQL Server Interview Questions

1.What is Trigger? What is its use? 
Triggers are a special class of stored procedure defined to execute automatically when an UPDATE, INSERT, or DELETE statement is issued against a table or view. Triggers are powerful tools that sites can use to enforce their business rules automatically when data is modified.

2. What are the types of Triggers?
The CREATE TRIGGER statement can be defined with the FOR UPDATE, FOR INSERT, or FOR DELETE clauses to target a trigger to a specific class of data modification actions. When FOR UPDATE is specified, the IF UPDATE (column_name) clause can be used to target a trigger to updates affecting a particular column.
You can use the FOR clause to specify when a trigger is executed:

•  AFTER (default) - The trigger executes after the statement that triggered it completes. If the statement fails with an error, such as a constraint violation or syntax error, the trigger is not executed. AFTER triggers cannot be specified for views.
•  INSTEAD OF -The trigger executes in place of the triggering action. INSTEAD OF triggers can be specified on both tables and views. You can define only one INSTEAD OF trigger for each triggering action (INSERT, UPDATE, and DELETE). INSTEAD OF triggers can be used to perform enhance integrity checks on the data values supplied in INSERT and UPDATE statements. INSTEAD OF triggers also let you specify actions that allow views, which would normally not support updates, to be updatable.

An INSTEAD OF trigger can take actions such as:

• Ignoring parts of a batch.
• Not processing a part of a batch and logging the problem rows.
• Taking an alternative action if an error condition is encountered.

3. What are the new kinds of triggers in sql 2000?
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.
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.

4. When should one use "instead of Trigger"? Example?
CREATE TABLE BaseTable
(
PrimaryKey int IDENTITY(1,1),
Color nvarchar(10) NOT NULL,
Material nvarchar(10) NOT NULL,
ComputedCol AS (Color + Material)
)
GO

--Create a view that contains all columns from the base table.
CREATE VIEW InsteadView
AS SELECT PrimaryKey, Color, Material, ComputedCol
FROM BaseTable
GO

--Create an INSTEAD OF INSERT trigger on tthe view.
CREATE TRIGGER InsteadTrigger on InsteadView
INSTEAD OF INSERT
AS
BEGIN
--Build an INSERT statement ignoring inserrted.PrimaryKey and
--inserted.ComputedCol.
INSERT INTO BaseTable
SELECT Color, Material
FROM inserted
END
GO

-- can insert value to basetable by this insert into basetable(color,material) values ('red','abc')

-- insert into InsteadView(color,material)) values ('red','abc') can't do this.
-- It will give error "'PrimaryKey' iin table 'InsteadView' cannot be null."

-- can insert value through table by this<
insert into InsteadView values (1,'red','abc',1) --PrimaryKey, ComputedCol wont take values from here

5. Difference between trigger and stored procedure?
Trigger will get execute automatically when an UPDATE, INSERT, or DELETE statement is issued against a table or view.
We have to call stored procedure manually, or it can execute automatic when the SQL Server starts (You can use the sp_procoption system stored procedure to mark the stored procedure to automatic execution when the SQL Server will start.

6. The following trigger generates an e-mail whenever a new title is added.
CREATE TRIGGER reminder
ON titles
FOR INSERT
AS
EXEC master..xp_sendmail 'MaryM', 'New title, mention in the next report to distributors.'

7. Drawback of trigger? Its alternative solution?
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.

STORED PROCEDURE | SQL Server Interview Questions

1. What is Stored procedure?
A stored procedure is a set of Structured Query Language (SQL) statements that you assign a name to and store in a database in compiled form so that you can share it between a number of programs.

•  They allow modular programming.
•  They allow faster execution.
•  They can reduce network traffic.
•  They can be used as a security mechanism.

2. What are the different types of Storage Procedure?

1.  Temporary Stored Procedures - SQL Server supports two types of temporary procedures: local and global. A local temporary procedure is visible only to the connection that created it. A global temporary procedure is available to all connections. Local temporary procedures are automatically dropped at the end of the current session. Global temporary procedures are dropped at the end of the last session using the procedure. Usually, this is when the session that created the procedure ends. Temporary procedures named with # and ## can be created by any user.
2.   System stored procedures- are created and stored in the master database and have the sp_ prefix.(or xp_) System stored procedures can be executed from any database without having to qualify the stored procedure name fully using the database name master. (If any user-created stored procedure has the same name as a system stored procedure, the user-created stored procedure will never be executed.)
3.  Automatically Executing Stored Procedures - One or more stored procedures can execute automatically when SQL Server starts. The stored procedures must be created by the system administrator and executed under the sysadmin fixed server role as a background process. The procedure(s) cannot have any input parameters.
4.  User stored procedure

3. How do I mark the stored procedure to automatic execution?
You can use the sp_procoption system stored procedure to mark the stored procedure to automatic execution when the SQL Server will start. Only objects in the master database owned by dbo can have the startup setting changed and this option is restricted to objects that have no parameters.

USE master

(EXEC sp_procoption 'indRebuild', 'startup', 'true')

4. How will know whether the SQL statements are executed?
When used in a stored procedure, the RETURN statement can specify an integer value to return to the calling application, batch, or procedure. If no value is specified on RETURN, a stored procedure returns the value 0. The stored procedures return a value of 0 when no errors were encountered. Any nonzero value indicates an error occurred.

5. Why one should not prefix user stored procedures with sp_?
It is strongly recommended that you do not create any stored procedures using sp_ as a prefix. SQL Server always looks for a stored procedure beginning with sp_ in this order:

•     The stored procedure in the master database.
•     The stored procedure based on any qualifiers provided (database name or owner).
•     The stored procedure using dbo as the owner, if one is not specified.

Therefore, although the user-created stored procedure prefixed with sp_ may exist in the current database, the master database is always checked first, even if the stored procedure is qualified with the database name.

6. What can cause a Stored procedure execution plan to become invalidated and/or fall out of cache?

•   Server restart
•   Plan is aged out due to low use
•   DBCC FREEPROCCACHE (sometime desired to force it)

7. When do one need to recompile stored procedure?
if a new index is added from which the stored procedure might benefit, optimization does not automatically happen (until the next time the stored procedure is run after SQL Server is restarted).

8. SQL Server provides three ways to recompile a stored procedure:

• The sp_recompile system stored procedure forces a recompile of a stored procedure the next time it is run.
• Creating a stored procedure that specifies the WITH RECOMPILE option in its definition indicates that SQL Server does not cache a plan for this stored procedure; the stored procedure is recompiled each time it is executed. Use the WITH RECOMPILE option when stored procedures take parameters whose values differ widely between executions of the stored procedure, resulting in different execution plans to be created each time. Use of this option is uncommon, and causes the stored procedure to execute more slowly because the stored procedure must be recompiled each time it is executed.
• You can force the stored procedure to be recompiled by specifying the WITH RECOMPILE option when you execute the stored procedure. Use this option only if the parameter you are supplying is atypical or if the data has significantly changed since the stored procedure was created.

9. How to find out which stored procedure is recompiling? How to stop stored procedures from recompiling?
10. I have Two Stored Procedures SP1 and SP2 as given below. How the Transaction works, whether SP2 Transaction succeeds or fails?

CREATE PROCEDURE SP1 AS

BEGIN TRAN

INSERT INTO MARKS (SID,MARK,CID) VALUES (5,6,3)

EXEC SP2

ROLLBACK

GO

CREATE PROCEDURE SP2 AS

BEGIN TRAN

INSERT INTO MARKS (SID,MARK,CID) VALUES (100,100,103)

commit tran

GO

Both will get roll backed.

CREATE PROCEDURE SP1 AS

BEGIN TRAN

    INSERT INTO MARKS (SID,MARK,CID) VALUES (5,6,3)

    BEGIN TRAN

        INSERT INTO STUDENT (SID,NAME1) VALUES (1,'SA')

    commit tran

ROLLBACK TRAN

GO

Both will get roll backed.

11. How will you handle Errors in Sql Stored Procedure?

INSERT NonFatal VALUES (@Column2)

IF @@ERROR <>0

 BEGIN

  PRINT 'Error Occured'

 END

http://www.sqlteam.com/item.asp?ItemID=2463

12. How will you raise an error in sql?
RAISERROR - Returns a user-defined error message and sets a system flag to record that an error has occurred. Using RAISERROR, the client can either retrieve an entry from the sysmessages table or build a message dynamically with user-specified severity and state information. After the message is defined it is sent back to the client as a server error message.
I have a stored procedure like
commit tran
create table a()
insert into table b
--
--
rollback tran
what will be the result? Is table created? data will be inserted in table b?

13. How you will return XML from Stored Procedure?
You use the FOR XML clause of the SELECT statement, and within the FOR XML clause you specify an XML mode: RAW, AUTO, or EXPLICIT.

14. Can a Stored Procedure call itself (recursive). If so then up to what level and can it be control?
Stored procedures are nested when one stored procedure calls another. You can nest stored procedures up to 32 levels. The nesting level increases by one when the called stored procedure begins execution and decreases by one when the called stored procedure completes execution. Attempting to exceed the maximum of 32 levels of nesting causes the whole calling stored procedure chain to fail. The current nesting level for the stored procedures in execution is stored in the @@NESTLEVEL function.

eg:

SET NOCOUNT ON

USE master

IF OBJECT_ID('dbo.sp_calcfactorial') IS NOT NULL

DROP PROC dbo.sp_calcfactorial

GO

CREATE PROC dbo.sp_calcfactorial

@base_number int, @factorial int OUT

AS

DECLARE @previous_number int

IF (@base_number<2) SET @factorial=1 -- Factorial of 0 or 1=1

ELSE BEGIN

SET @previous_number=@base_number-1

EXEC dbo.sp_calcfactorial @previous_number, @factorial OUT -- Recursive call

IF (@factorial=-1) RETURN(-1) -- Got an error, return

SET @factorial=@factorial*@base_number

END

RETURN(0)

GO

calling proc.

DECLARE @factorial int

EXEC dbo.sp_calcfactorial 4, @factorial OUT

SELECT @factorial

15. What is Nested Triggers?
Triggers are nested when a trigger performs an action that initiates another trigger, which can initiate another trigger, and so on. Triggers can be nested up to 32 levels, and you can control whether triggers can be nested through the nested triggers server configuration option.

16. 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.

17. Difference between view and stored procedure?
Views can have only select statements (create, update, truncate, delete statements are not allowed) Views cannot have “select into”, “Group by” “Having”, ”Order by”

18. What is a Function & what are the different user defined functions?
Function is a saved Transact-SQL routine that returns a value. User-defined functions cannot be used to perform a set of actions that modify the global database state. User-defined functions, like system functions, can be invoked from a query. They also can be executed through an EXECUTE statement like stored procedures.
    i)Scalar Functions
Functions are scalar-valued if the RETURNS clause specified one of the scalar data types
    ii)Inline Table-valued Functions
If the RETURNS clause specifies TABLE with no accompanying column list, the function is an inline function.
    iii)Multi-statement Table-valued Functions
If the RETURNS clause specifies a TABLE type with columns and their data types, the function is a multi-statement table-valued function.

19. What are the difference between a function and a stored procedure?
•    Functions can be used in a select statement where as procedures cannot
•    Procedure takes both input and output parameters but Functions takes only input parameters
•    Functions cannot return values of type text, ntext, image & timestamps where as procedures can
•    Functions can be used as user defined datatypes in create table but procedures cannot
Eg:- create table <tablename>(name varchar(10),salary getsal(name))
Here getsal is a user defined function which returns a salary type, when table is created no storage is allotted for salary type, and getsal function is also not executed, But when we are fetching some values from this table, getsal function get’s executed and the return
Type is returned as the result set.

JOINS | SQL Server Interview Questions

1. What are joins?
Sometimes we have to select data from two or more tables to make our result complete. We have to perform a join.

2. How many types of Joins?
Joins can be categorized as:

•  Inner joins: (the typical join operation, which uses some comparison operator like = or <>). These include equi-joins and natural joins.
• Inner joins use a comparison operator to match rows from two tables based on the values in common columns from each table. For example, retrieving all rows where the student identification number is the same in both the students and courses tables.
•  Outer joins: Outer joins can be a left, a right, or full outer join. Outer joins are specified with one of the following sets of keywords when they are specified in the FROM clause:
•   LEFT JOIN or LEFT OUTER JOIN -The result set of a left outer join includes all the rows from the left table specified in the LEFT OUTER clause, not just the ones in which the joined columns match. When a row in the left table has no matching rows in the right table, the associated result set row contains null values for all select list columns coming from the right table.
• RIGHT JOIN or RIGHT OUTER JOIN - A right outer join is the reverse of a left outer join. All rows from the right table are returned. Null values are returned for the left table any time a right table row has no matching row in the left table.
•  FULL JOIN or FULL OUTER JOIN - A full outer join returns all rows in both the left and right tables. Any time a row has no match in the other table, the select list columns from the other table contain null values. When there is a match between the tables, the entire result set row contains data values from the base tables.
•   Cross joins: Cross joins return all rows from the left table, each row from the left table is combined with all rows from the right table. Cross joins are also called Cartesian products. (A Cartesian join will get you a Cartesian product. A Cartesian join is when you join every row of one table to every row of another table. You can also get one by joining every row of a table to every row of itself.)

3. What is self join?
A table can be joined to itself in a self-join.

4. What are the differences between UNION and JOINS?
A join selects columns from 2 or more tables. A union selects rows.

5. Can I improve performance by using the ANSI-style joins instead of the old-style joins?
Code Example 1:
select o.name, i.name
from sysobjects o, sysindexes i
where o.id = i.id
Code Example 2:
select o.name, i.name
from sysobjects o inner join sysindexes i
on o.id = i.id
You will not get any performance gain by switching to the ANSI-style JOIN syntax.
Using the ANSI-JOIN syntax gives you an important advantage: Because the join logic is cleanly separated from the filtering criteria, you can understand the query logic more quickly.
The SQL Server old-style JOIN executes the filtering conditions before executing the joins, whereas the ANSI-style JOIN reverses this procedure (join logic precedes filtering).
Perhaps the most compelling argument for switching to the ANSI-style JOIN is that Microsoft has explicitly stated that SQL Server will not support the old-style OUTER JOIN syntax indefinitely. Another important consideration is that the ANSI-style JOIN supports query constructions that the old-style JOIN syntax does not support.

6.What is derived table?
Derived tables are SELECT statements in the FROM clause referred to by an alias or a user-specified name. The result set of the SELECT in the FROM clause forms a table used by the outer SELECT statement. For example, this SELECT uses a derived table to find if any store carries all book titles in the pubs database:
SELECT ST.stor_id, ST.stor_name
FROM stores AS ST,
     (SELECT stor_id, COUNT(DISTINCT title_id) AS title_count
      FROM sales
      GROUP BY stor_id
     ) AS SA
WHERE ST.stor_id = SA.stor_id
AND SA.title_count = (SELECT COUNT(*) FROM titles)

DATA TYPES | SQL Server Interview Questions

1. What are the data types in SQL?

2. Difference between char and nvarchar / char and varchar data-type?

char[(n)] - Fixed-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is n bytes. The SQL-92 synonym for char is character. nvarchar(n) - Variable-length Unicode character data of n characters. n must be a value from 1 through 4,000. Storage size, in bytes, is two times the number of characters entered.  The data entered can be 0 characters in length. The SQL-92 synonyms for nvarchar are national char varying and national character varying. varchar[(n)] - Variable-length non-Unicode character data with length of n bytes. n must be a value from 1 through 8,000. Storage size is the actual length in bytes of the data entered, not n bytes. The data entered can be 0 characters in length. The SQL-92 synonyms for varchar are char varying or character varying.

3. GUID datasize?

128bit

4. How GUID becoming unique across machines?

To ensure uniqueness across machines, the ID of the network card is used (among others) to compute the number.

5. What is the difference between text and image data type?

Text and image. Use text for character data if you need to store more than 255 characters in SQL Server 6.5, or more than 8000 in SQL Server 7.0. Use image for binary large objects (BLOBs) such as digital images. With text and image data types, the data is not stored in the row, so the limit of the page size does not apply.All that is stored in the row is a pointer to the database pages that contain the data.Individual text, ntext, and image values can be a maximum of 2-GB, which is too long to store in a single data row.

INDEX | SQL Server Interview Questions

1. What is Index? It’s purpose?

Indexes in databases are similar to indexes in books. In a database, an index allows the database program to find data in a table without scanning the entire table.  An index in a database is a list of values in a table with the storage locations of rows in the table that contain each value. Indexes can be created on either a single
column or a combination of columns in a table and are implemented in the form of B-trees. An index contains an entry with one or more columns (the search key) from each row in a table. A B-tree is sorted on the search key, and can be searched efficiently on any leading subset of the search key. For example, an index on columns A, B, C can be searched efficiently on A, on A, B, and A, B, C.

2. Explain about Clustered and non clustered index? How to choose between a Clustered Index and a Non-Clustered Index?

There are clustered and nonclustered indexes. A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table can have only one clustered index. The leaf nodes of a clustered index contain the data pages. A nonclustered 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 on disk. The leaf nodes of a nonclustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.
Consider using a clustered index for:

•  Columns that contain a large number of distinct values.
•  Queries that return a range of values using operators such as BETWEEN, >, >=, <, and <=.
•  Columns that are accessed sequentially.
•  Queries that return large result sets.

Non-clustered indexes have the same B-tree structure as clustered indexes, with two significant differences:

•  The data rows are not sorted and stored in order based on their non-clustered keys.
•  The leaf layer of a non-clustered index does not consist of the data pages. Instead, the leaf nodes     contain  index rows. Each index row contains the non-clustered key value and one or more row locators that point to the data row (or rows if the index is not unique) having the key value.
• Per table only 249 non clustered indexes.

3. Disadvantage of index?

Every index increases the time in takes to perform INSERTS, UPDATES and DELETES, so the number of indexes should not be very much.

4.Given a scenario that I have a 10 Clustered Index in a Table to all their 10 Columns. What are the advantages and disadvantages?

A: Only 1 clustered index is possible.

5. How can I enforce to use particular index?

You can use index hint (index=<index_name>) after the table name.
SELECT au_lname FROM authors (index=aunmind)

6. What is Index Tuning?

  One of the hardest tasks facing database administrators is the selection of appropriate columns for  non-clustered indexes. You should consider creating non-clustered indexes on any columns that are frequently referenced in the WHERE clauses of SQL statements. Other good candidates are columns referenced by JOIN and GROUP BY operations.
  You may wish to also consider creating non-clustered indexes that cover all of the columns used by certain frequently issued queries. These queries are referred to as “covered queries” and experience excellent performance gains. Index Tuning is the process of finding appropriate column for non-clustered indexes.
SQL Server provides a wonderful facility known as the Index Tuning Wizard which greatly enhances the index selection process.

7. Difference between Index defrag and Index rebuild?

    When you create an index in the database, the index information used by queries is stored in index pages. The sequential index pages are chained together by pointers from one page to the next. When changes are made to the data that affect the index, the information in the index can become scattered in the database. Rebuilding an index reorganizes the storage of the index data (and table data in the case of a clustered index) to remove fragmentation. This can improve disk performance by reducing the number of page reads required to  obtain the requested data DBCC INDEXDEFRAG - Defragments clustered and secondary indexes of the specified table or view.

8. What is sorting and what is the difference between sorting & clustered indexes?

The ORDER BY clause sorts query results by one or more columns up to 8,060 bytes. This will happen by the time when we retrieve data from database. Clustered indexes physically sorting data, while inserting/updating the table.

9. 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

10. What is fillfactor? What is the use of it ? What happens when we ignore it? When you should use low fill factor?

When you create a clustered index, the data in the table is stored in the data pages of the database according to the order of the values in the indexed columns. When new rows of data are inserted into the table or the values in the indexed columns are changed, Microsoft® SQL Server™ 2000 may have to reorganize the storage of the data in the table to make room for the  new row and maintain the ordered storage of the data. This also applies to nonclustered indexes. When data is added or changed, SQL Server may have to reorganize the storage of the data in the nonclustered index pages. When a new row is added to a full index page, SQL Server moves approximately half the rows to a new page to make room for the new row. This reorganization is known as a page split. Page splitting can impair performance and fragment the storage of the data in a table.
    When creating an index, you can specify a fill factor to leave extra gaps and reserve a percentage of free space on each leaf level page of the index to accommodate future expansion in the storage of the table's data and reduce the potential for page splits. The fill factor value is a percentage from 0 to 100 that specifies how much to fill the data pages after the index is created. A value of 100 means the pages will be full and will take the least amount of storage space. This setting should be used only when there will be no changes to the data,
for example, on a read-only table. A lower value leaves more empty space on the data pages, which reduces the need to split data pages as indexes grow but requires more storage space.
This setting is more appropriate when there will be changes to the data in the table.

SQL Server Interview Questions and answers for experienced pdf

Topic wise Interview Questions:

BASIC Concepts
INDEX

DATA TYPES

JOINS

STORED PROCEDURE

TRIGGER

LOCK

VIEW

TRANSACTION

TOOLS 
PERMISSIONS

XML

Transact-SQL Optimization Tips 

T-SQL Queries 

Application Domains | VB.Net Tutorial For Biginners PDF Download

Introduction
   Traditionally when many applications are executed on the same machine, they are isolated by something known as process. Ideally each application is loaded in its own process also known as address space. This isolation is need so that these applications do not tamper with other applications either intentionally or accidentally. Isolating applications is also important for application security. For example, one can run controls
from several Web applications in a single browser process in such a way that the controls cannot access each other's data and resources.
      There are however many instances in which one would like an application to have the ability to communicate with other applications. Since these applications are loaded into different address spaces, there must be some form of context switching needed to allow one application to communicate with another. Inter process communication has to rely on operating systems support to manage this context switching and it is generally an expensive operation. Context switching means saving a process's context, it could also mean swapping the process out to virtual memory (to the page file stored on disk). If a single machine has a large number of active processes, the CPU is often reduced to swapping processes in and out of memory continuously, a phenomenon known as thrashing.

Application Domains
   There should be a method by which the different applications can be executed in isolation from each other and which would also allow these applications to communicate with each other in a better way than offered by context switching. Microsoft .NET has introduced the Application Domain concept for precisely this reason. Application domains provide a secure and versatile unit of processing that the CLR can use to provide isolation between applications. Further, one can specify custom security policies on an Application Domain
to ensure that codes run in an extremely strict and controlled app domain. One can run several application domains in a single process with the same level of isolation that would exist in separate processes, but without incurring the additional overhead of context switching between the processes. In Microsoft .NET, code normally passes a verification process before it can be executed. This code is considered as type-safe code
and this allows CLR to provide a great level of isolation at the process level. Type-safe codes have less chances of causing memory faults.
    Code running in one application should not directly access code or resources from another application. The CLR enforces this isolation by preventing direct calls between objects in different application domains. Objects that pass between domains are either copied or accessed by proxy. If the object is copied, the call to the object is local. That is, both the caller and the object being referenced are in the same application domain. If the object is accessed through a proxy, the call to the object is remote. In this case, the caller and the object being referenced are in different application domains. As such, the metadata for the object being referenced must be available to both application domains to allow the method call to be JIT-compiled properly.

Application Domains And Assemblies
Before an assembly can be executed, it must be loaded into an application domain. By default, the CLR loads an assembly into an application domain containing the code that references it. In this way the assembly’s data and code are isolated to the application using it. In case, multiple application domains reference an assembly, the assembly’s code is shared amongst the different application domains. Such an assembly is said to be
domain-neutral. An assembly is not shared between domains when it is granted a different set of permissions in each domain. This can occur if the runtime host sets an application domain-level security policy. Assemblies should not be loaded as domainneutral if the set of permissions granted to the assembly is to be different in each domain.

Programming with Application Domains
    Application domains are normally automatically created and managed by runtime hosts. However, Microsoft .NET also provides control to application developers to create and manage their own application domains. This would allow the developers to have control over loading and unloading the assemblies in different domains for performance reasons and maintain a high degree of isolation. Note that individual assemblies cannot be unloaded, the entire app domain has to be unloaded.
   If development is being carried out with some code or components downloaded from the Internet, running it in its own application domain provides an excellent way to isolate the rest of the applications from this code.
   The System namespace contains the class AppDomain. This class contains methods to create an application domain, to load and unload assemblies in the application domain.
    An example will be useful to illustrate how application domains can be created and assemblies loaded and unloaded into the application domains. Note that only those assemblies that have been declared as Public can be loaded at runtime.
   Copy and paste the following code into Notepad and save it as Display.cs/Display.vb, depending on the programming language used.

Display.vb
Code Listing in C#
using System;
public class Display
{
public static void Main()
{
Console.WriteLine("This is written by assembly 1");
Console.ReadLine();
}
}
Compiling in C#
Compilation csc /r:System.dll Display.cs
Code Listing in VB.NET
Imports System
Public Module Display
Sub Main()
Console.WriteLine("This is written by assembly 1")
Console.ReadLine()
End Sub
End Module
Compiling in VB.NET
vbc /r:System.dll Display.vb
Similarly, copy and paste the following code into Notepad and save it as Display2.cs/Display2.vb, depending on the programming language used. Display2.cs
Code Listing in C#
Imports System
Public Module Display
Sub Main()
Console.WriteLine("This is written by assembly 2")
Console.ReadLine()
End Sub
End Module
Compiling in C#
Compilation csc /r:System.dll Display2.cs
Code Listing in VB.NET
Imports System
Public Module Display
Sub Main()
Console.WriteLine("This is written by assembly 2")
Console.ReadLine()
End Sub
End Module
Compiling in VB.NET
'Compilation vbc /r:System.dll Display2.vb
The following code - CreateAppDomain.cs/CreateAppDomain.vb contains the following code that shows how to create an application domain programmatically and how to load assemblies during runtime.
Code Listing in C#
using System;
using System.Reflection;
public class CreateAppDomain
{
AppDomain m_objAppDomain;
public static void Main()
{
String strAppDomainName1 ;
String strAppDomainName2 ;
String strAssemblyToBeExecuted ;
strAppDomainName1 = "TestAppDomain1";
strAppDomainName2 = "TestAppDomain2";
strAssemblyToBeExecuted = "Display.exe";
CreateApplicationDomain(strAppDomainName1, strAssemblyToBeExecuted);
AppDomain.Unload(m_objAppDomain);
strAssemblyToBeExecuted = "Display2.exe" ;
CreateApplicationDomain(strAppDomainName2, strAssemblyToBeExecuted);
AppDomain.Unload(m_objAppDomain);
}
private void CreateApplicationDomain(String p_strAppDomainName , String
p_strAssemblyToBeExecuted)
{
try
{
m_objAppDomain = AppDomain.CreateDomain(p_strAppDomainName);
m_objAppDomain.ExecuteAssembly(p_strAssemblyToBeExecuted);
}
catch(AppDomainUnloadedException objException )
{
Console.WriteLine("Unable to create application domain");
Console.WriteLine("Exception is " + objException.toString());
}
}
}
Compiling in C#
Compilation csc /r:System.dll CreateAppDomain.cs
Code Listing in VB.NET
Imports System
Imports System.Reflection
Module CreateAppDomain
Dim m_objAppDomain As AppDomain
Sub Main()
Dim strAppDomainName1 As String
Dim strAppDomainName2 As String
Dim strAssemblyToBeExecuted As String
strAppDomainName1 = "TestAppDomain1"
strAppDomainName2 = "TestAppDomain2"
strAssemblyToBeExecuted = "Display.exe"
CreateApplicationDomain(strAppDomainName1, strAssemblyToBeExecuted)
AppDomain.Unload(m_objAppDomain)
strAssemblyToBeExecuted = "Display2.exe"
CreateApplicationDomain(strAppDomainName2, strAssemblyToBeExecuted)
AppDomain.Unload(m_objAppDomain)
End Sub
Private Sub CreateApplicationDomain(p_strAppDomainName As String,
p_strAssemblyToBeExecuted As String)
Try
m_objAppDomain = AppDomain.CreateDomain(p_strAppDomainName)
m_objAppDomain.ExecuteAssembly(p_strAssemblyToBeExecuted)
Catch objException As AppDomainUnloadedException
Console.WriteLine("Unable to create application domain")
Console.WriteLine("Exception is " & objException.toString())
End Try
End Sub
End Module
Compiling in VB.NET
Compilation vbc /r:System.dll CreateAppDomain.vb

Shared Assemblies | VB.Net Tutorial For Biginners PDF Download

  A shared assembly is an assembly available for use by multiple applications on the  computer. To make the assembly global, it has to be put into the Global Assembly Cache. Each computer where the common language runtime is installed has a machine-wide code cache called the global assembly cache. The global assembly cache stores assemblies specifically for sharing by several applications on the computer.
   You should share assemblies by installing them into the global assembly cache only when you need to. As a general guideline, keep assembly dependencies private and locate assemblies in the application directory unless sharing an assembly is explicitly required.
   This is achieved with the help of a global assembly cache tool (gacutil.exe) provided by the .NET Framework. One can also drag & drop the assemblies into the Global Assembly Cache directory.
    However, when an assembly has to be put into the Global Assembly Cache it needs to be signed with a strong name. A strong name contains the assembly's identity i.e. it’s text name, version number, and culture information strengthened by a public key and a digital signature generated over the assembly. This is because the CLR verifies the strong name signature when the assembly is placed in the Global Assembly Cache.

Private Assemblies | VB.Net Tutorial For Biginners PDF Download

    A private assembly is an assembly that is deployed with an application and is available only for that application. That is, other applications do not share the private assembly. Private assemblies are installed in a folder of the application's directory structure. Typically, this is the folder containing the application's executable file.
   For most .NET Framework applications, you keep the assemblies that make up an application in the application's directory, in a subdirectory of the application's directory. You can override where the CLR looks for an assembly by using the <codeBase> element in a configuration file.