SQL SERVER TRIGGERS

What is a Trigger

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. You can’t explicitly invoke triggers. 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.

After TriggersS Example

CREATE TRIGGER trgAfterDelete ON [dbo].[Employee_Test]

AFTER DELETE/INSERT/UPDAT

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,Audit_Action,Audit_Timestamp)

values(@empid,@empname,@empsal,@audit_action,getdate());

PRINT 'AFTER DELETE TRIGGER fired

(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

INSTEAD OF TRIGGERS EXAMPLES

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,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp)

values(@emp_id,@emp_name,@emp_sal,'Deleted -- Instead Of Delete Trigger.',getdate());

PRINT 'Record Deleted -- Instead Of Delete Trigger.'

end

END

GO