Thursday, September 22

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

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