Triggers -- Sql Server

   A trigger is a special kind of a store procedure that executes in response to certain action on the table like insertion, deletion or updating 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 on 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:

• After Triggers (For Triggers)  
• Instead Of Triggers 

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, Audit_Action, Audit_Timestamp)

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

          PRINT 'AFTER INSERT trigger fired.'

GO

The CREATE TRIGGER statement is used to create the trigger. THE ON clause specifies the table name on which the trigger is to be attached. The FOR INSERT specifies that this is an AFTER INSERT trigger. In place of FOR INSERT, AFTER INSERT can be used. Both of them mean the same. 

In the trigger body, table named inserted has been used. This table is a logical table and contains the row that has been inserted. I have selected the fields from the logical inserted table from the row that has been inserted into different variables, and finally inserted those values into the Audit table.

AFTER UPDATE Trigger:

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

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

The AFTER UPDATE Trigger is created in which the updated record is inserted into the audit table. There is no logical table updated like the logical table inserted. We can obtain the updated value of a field from the update (column_name) function.

 In our trigger, we have used, if update (Emp_Name) to check if the column Emp_Name has been updated. We have similarly checked the column Emp_Sal for an update. 

AFTER DELETE Trigger:

This trigger is fired after a delete on the table. Let’s create the trigger as: 

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, Audit_Action, Audit_Timestamp)

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

          PRINT 'AFTER DELETE TRIGGER fired.'

GO

In this trigger, the deleted record’s data is picked from the logical deleted table and inserted into the audit table.

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 an Instead 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: 

INSTEAD OF INSERT Trigger.

INSTEAD OF UPDATE Trigger.

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,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

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.