Triggers -- Sql Server

Triggers -- Sql Server

What is a Trigger

1. A trigger is a special kind of a store procedure that executes in response to certain action on the table like insertion, deletion or updation of data.
2.  It is a database object which is bound to a table and is executed automatically.
3.  You can’t explicitly invoke triggers. 
4. The only way to do this is by performing the required action no the table that they are assigned to.

Types Of Triggers

There are three action query types that you use in SQL which are INSERT, UPDATE and DELETE. So, there are three types of triggers and hybrids that come from mixing and matching the events and timings that fire them.

Basically, triggers are classified into two main types:- 

(i) After Triggers (For Triggers) 

(ii) Instead Of Triggers 

(i) After Triggers

These triggers run after an insert, update or delete on a table. They are not supported for views. 

AFTER TRIGGERS can be classified further into three types as: 

(a) AFTER INSERT Trigger. 

(b) AFTER UPDATE Trigger. 

(c) AFTER DELETE Trigger. 

(a) AFTRE INSERT Trigger

CREATE TABLE Employee_Test
(
Emp_ID INT Identity,
Emp_name Varchar(100),
Emp_Sal Decimal (10,2)
)

INSERT INTO Employee_Test VALUES ('Anees',1000);
INSERT INTO Employee_Test VALUES ('Rick',1200);
INSERT INTO Employee_Test VALUES ('John',1100);
INSERT INTO Employee_Test VALUES ('Stephen',1300);
INSERT INTO Employee_Test VALUES ('Maria',1400);

CREATE TABLE Employee_Test_Audit
(
Emp_ID int,
Emp_name varchar(100),
Emp_Sal decimal (10,2),
Audit_Action varchar(100),
Audit_Timestamp datetime
)

This trigger is fired after an INSERT on the table. Let’s create the trigger as:-

CREATE TRIGGER trgAfterInsert ON [dbo].[Employee_Test] 
FOR INSERT
AS
 declare @empid int;
 declare @empname varchar(100);
 declare @empsal decimal(10,2);
 declare @audit_action varchar(100);

 select @empid=i.Emp_ID from inserted i; 
 select @empname=i.Emp_Name from inserted i; 
 select @empsal=i.Emp_Sal from inserted i; 
 set @audit_action='Inserted Record -- After Insert Trigger.';

 insert into Employee_Test_Audit
           (Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp) 
 values(@empid,@empname,@empsal,@audit_action,getdate());

 PRINT 'AFTER INSERT trigger fired.'
GO

To see the newly created trigger in action, lets insert a row into the main table as : 

insert into Employee_Test values('Chris',1500);