Trigger with example

A trigger is a special kind of stored procedure that automatically executes when an event occurs in the database server.

DML triggers execute when a user tries to modify data through a data manipulation language (DML) event.

Syntex:
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 }

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.

For e.g
USE AdventureWorks2008R2;
GO
IF OBJECT_ID ('Sales.reminder2','TR') IS NOT NULL
    DROP TRIGGER Sales.reminder2;
GO
CREATE TRIGGER reminder2
ON Sales.Customer
AFTER INSERT, UPDATE, DELETE
AS
   EXEC msdb.dbo.sp_send_dbmail
        @profile_name = 'AdventureWorks2008R2 Administrator',
        @recipients = 'danw@Adventure-Works.com',
        @body = 'Don''t forget to print a report for the sales force.',
        @subject = 'Reminder';
GO