Triggers in SQL SERVER.

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This is an article on Triggers in SQL SERVER. in SQL Server.
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A trigger is a special kind of stored procedure that is invoked whenever an attempt is made to modify the data in the table it protects. Modifications to the table are made ussing INSERT,UPDATE,OR DELETE statements.Triggers are used to enforce data integrity and business rules such as automatically updating summary data. It allows to perform cascading delete or update operations. If constraints exist on the trigger table,they are checked prior to the trigger execution. If constraints are violated statement will not be executed and trigger will not run.Triggers are associated with tables and they are automatic . Triggers are automatically invoked by SQL SERVER. Triggers prevent incorrect , unauthorized,or inconsistent changes to data.

Creation of Triggers



Triggers are created with the CREATE TRIGGER statement. This statement specifies that the on which table trigger is defined and on which events trigger will be invoked.

To drop Trigger one can use DROP TRIGGER statement.

Code: SQL
CREATE TRIGGER [owner.]trigger_name

ON[owner.] table_name

FOR[INSERT/UPDATE/DELETE] AS

IF UPDATE(column_name)

[{AND/OR} UPDATE(COLUMN_NAME)...]

{ sql_statements }

Trigger rules and guidelines



A table can have only three triggers action per table : UPDATE ,INSERT,DELETE. Only table owners can create and drop triggers for the table.This permission cannot be transferred.A trigger cannot be created on a view or a temporary table but triggers can reference them. A trigger should not include SELECT statements that return results to the user, because the returned results would have to be written into every application in which modifications to the trigger table are allowed. They can be used to help ensure the relational integrity of database.On dropping a table all triggers associated to the triggers are automatically dropped .


The system stored procedure sp_depends can be used to find out which tables have trigger on them. Following sql statements are not allowed in a trigger they are:-

Code: SQL
ALL CREATE statements,ALL DROP statements,ALTER TABLE AND ALTER DATABASE,TRUNCATE TABLE,GRANT AND REVOKE,UPDATE STATISTICS,RECONFIGURE,LOAD DATABASE AND LOAD TRANSACTION, ALL DISK statements,SELECT INTO


INSERT trigger



When an INSERT trigger statement is executed ,new rows are added to the trigger table and to the inserted table at the same time. The inserted table is a logical table that holds a copy of rows that have been inserted. The inserted table can be examined by the trigger ,to determine whether or how the trigger action are carried out.

The inserted table allows to compare the INSERTED rows in the table to the rows in the inserted table.The inserted table are always duplicates of one or more rows in the trigger table.With the inserted table ,inserted data can be referenced without having to store the information to the variables.

DELETE trigger



When a DELETE trigger statement is executed ,rows are deleted from the table and are placed in a special table called deleted table.

UPDATE trigger



When an UPDATE statement is executed on a table that has an UPDATE trigger,the original rows are moved into deleted table,While the update row is inserted into inserted table and the table is being updated.

Syntax
Code: SQL
IF UPDATE<column_name>
CREATE TRIGGER trigger1 ON member
FOR UPDATE
AS
IF UPDATE(last_name)
BEGIN
RAISEERROR('Transaction cannot be processed')
ROLLBACK TRANSACTION
END

Multi-row trigger



A multi-row insert can occur from an INSERT with a SELECT statement.Multirow considerations can also apply to multi-row updates and multi-row deletes.

Code: SQL
CREATE TRIGGER adult_insert ON adult
FOR INSERT
AS
DECLARE @rcnt int
SELECT @rcnt = @@rowcount
IF(SELECT COUNT(*) FROM member,inserted WHERE member.mem_num = inserted.mem_num)=0
BEGIN
PRINT 'Transaction not processed'
PRINT 'No entry for this member'
ROLLBACK TRANSACTION
END
IF(SELECT COUNT(*) FROM member,inserted WHERE member.mem_num = inserted.mem_num)<> @rcnt

BEGIN
PRINT 'Not all adults have an entry in the member table'
PRINT 'Multi-row insert transaction has been rolled backed'
ROLLBACK TRANSACTION
END
0
shabbir's Avatar, Join Date: Jul 2004
Go4Expert Founder
Nomination for article of the month for December 2008 Started.
0
phone00x's Avatar
Light Poster
dear friend'sSQL Server is easy to create trigger, so think.
than'k friend's
0
LenoxFinlay's Avatar
Banned
Hi,

Thanks for your useful information. For more information about this topic please visit <<snip>>. I think its more useful for you.
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itsys's Avatar
Banned
Nice Information for triger
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technica's Avatar, Join Date: Dec 2007
CoderByNature
A multi row-Trigger is useful part for me. Thanks for sharing the information with us all.
0
kumarmannu's Avatar
Banned
Great information please tell something about view.
0
TPrice's Avatar, Join Date: Aug 2010
Light Poster
Thanks for this information on triggers. I have been a little bit confused in this area as to their purpose and function, but I think that I have a better grasp of them now. The only question now is, do I understand this well enough to actually put it to work? I guess I will find out soon enough.
like this
0
rkjarvis's Avatar, Join Date: Jun 2011
Newbie Member
We have an application where rows are mysteriously deleted off a single table. Vendor claims it isn't their code and application folks claim they didn't do it. I used a delete trigger to insert the data being deleted to another table. When the delete inadvertantly happens, the deleted records are at least logged somewhere. The application team has a quick way to restore the missing data back into the original table. Is there a way to capture what ID is actually performing the delete in the trigger?
0
gsuryaprasath's Avatar, Join Date: Jul 2011
Newbie Member
Very nice explanation about ..its really superb..keep posting like these..

Last edited by shabbir; 12Jul2011 at 13:18.. Reason: Confine Links to Signatures