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. It is a database object which is bound to a table and is executed automatically.
There are three type of query that you use in SQL which are INSERT, UPDATE and DELETE.
Basically, triggers are classified into two main types:
1. After Triggers (For Triggers)
2. 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:
AFTER TRIGGERS can be classified further into three types as:
a. AFTER INSERT Trigger.
b. AFTER UPDATE Trigger.
c. AFTER DELETE Trigger.
Lets create a table and insert some records into that
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);
Now, create the audit table as:-
CREATE TABLE Employee_Test_Audit
(
Emp_ID int,
Emp_name varchar(100),
Emp_Sal decimal (10,2),
Audit_Action varchar(100),
Audit_Timestamp datetime
)
(a) After Insert Trigger
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,
values(@empid,@empname,@empsal,@audit_action,getdate());
PRINT 'AFTER INSERT trigger fired.'
GO
Insert record into table :
insert into Employee_Test values('Chris',1500);
now read the data from “Employee_Test_Audit “ table, you will see the record into that table as well.
AFTER UPDATE Trigger
CREATE TRIGGER trgAfterUpdate ON [dbo].[Employee_Test]
FOR UPDATE
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;
if update(Emp_Name)
set @audit_action='Updated Record -- After Update Trigger.';
if update(Emp_Sal)
set @audit_action='Updated Record -- After Update 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 UPDATE Trigger fired.'
GO
AFTER DELETE Trigger
CREATE TRIGGER trgAfterDelete ON [dbo].[Employee_Test]
AFTER DELETE
AS
declare @empid int;
declare @empname varchar(100);
declare @empsal decimal(10,2);
declare @audit_action varchar(100);
select @empid=d.Emp_ID from deleted d;
select @empname=d.Emp_Name from deleted d;
select @empsal=d.Emp_Sal from deleted d;
set @audit_action='Deleted -- After Delete Trigger.';
insert into Employee_Test_Audit
(Emp_ID,Emp_Name,Emp_Sal,
values(@empid,@empname,@empsal,@audit_action,getdate());
PRINT 'AFTER DELETE TRIGGER fired.'
GO
All the triggers can be enabled/disabled on the table using the statement
ALTER TABLE Employee_Test {ENABLE|DISBALE} TRIGGER ALL
Specific Triggers can be enabled or disabled as:
ALTER TABLE Employee_Test DISABLE TRIGGER trgAfterDelete
(ii) Instead Of Triggers
These can be used as an interceptor for anything that anyone tried to do on our table or view. If you define anInstead Of trigger on a table for the Delete operation, they try to delete rows, and they will not actually get deleted (unless you issue another delete instruction from within the trigger)
INSTEAD OF TRIGGERS can be classified further into three types as:
a. INSTEAD OF INSERT Trigger.
b. INSTEAD OF UPDATE Trigger.
c. INSTEAD OF DELETE Trigger.
Let’s create an Instead Of Delete Trigger as:
CREATE TRIGGER trgInsteadOfDelete ON [dbo].[Employee_Test]
INSTEAD OF DELETE
AS
declare @emp_id int;
declare @emp_name varchar(100);
declare @emp_sal int;
select @emp_id=d.Emp_ID from deleted d;
select @emp_name=d.Emp_Name from deleted d;
select @emp_sal=d.Emp_Sal from deleted d;
BEGIN
if(@emp_sal>1200)
begin
RAISERROR('Cannot delete where salary > 1200',16,1);
ROLLBACK;
end
else
begin
delete from Employee_Test where Emp_ID=@emp_id;
COMMIT;
insert into Employee_Test_Audit(Emp_ID,
values(@emp_id,@emp_name,@emp_
PRINT 'Record Deleted -- Instead Of Delete Trigger.'
end
END
GO
This trigger will prevent the deletion of records from the table where Emp_Sal > 1200. If such a record is deleted, the Instead Of Trigger will rollback the transaction, otherwise the transaction will be committed. Now, let’s try to delete a record with the Emp_Sal >1200 as:
delete from Employee_Test where Emp_ID=4
This will print an error message as defined in the RAISE ERROR statement as:
Server: Msg 50000, Level 16, State 1, Procedure trgInsteadOfDelete, Line 15
Cannot delete where salary > 1200
And this record will not be deleted.
