Instead of Trigger and After Trigger in SQL Server

Triggers are special type of stored procedure that automatically execute when a DDL or DML statement associated with the trigger is executed. DML Triggers are used to evaluate data after data manipulation using DML statements. We have two types of DML triggers.

Types of DML Triggers

1. After Trigger (using FOR/AFTER CLAUSE)

   This trigger fires after SQL Server completes the execution of the action successfully that fired it.
   Example :If you insert record/row in a table then the trigger associated with the insert event on this table will fire only after the row passes all the checks, such as primary key, rules, and constraints. If the record/row insertion fails, SQL Server will not fire the After Trigger.

2. Instead of Trigger (using INSTEAD OF CLAUSE)

   This trigger fires before SQL Server starts the execution of the action that fired it. This is much more different from the AFTER trigger, which fires after the action that caused it to fire. We can have an INSTEAD OF insert/update/delete trigger on a table that successfully executed but does not include the actual insert/update/delet to the table.
   Example :If you insert record/row in a table then the trigger associated with the insert event on this table will fire before the row passes all the checks, such as primary key, rules, and constraints. If the record/row insertion fails, SQL Server will fire the Instead of Trigger.

Example

 -- First create table Employee_Demo
CREATE TABLE Employee_Demo
(
 Emp_ID int identity,
 Emp_Name varchar(55),
 Emp_Sal decimal (10,2)
)
-- Now Insert records 
Insert into Employee_Demo values ('Amit',1000);
Insert into Employee_Demo values ('Mohan',1200);
Insert into Employee_Demo values ('Avin',1100);
Insert into Employee_Demo values ('Manoj',1300);
Insert into Employee_Demo values ('Riyaz',1400);
--Now create table Employee_Demo_Audit for logging/backup purpose of table Employee_Demo create table Employee_Demo_Audit
(
 Emp_ID int,
 Emp_Name varchar(55),
 Emp_Sal decimal(10,2),
 Audit_Action varchar(100),
 Audit_Timestamp datetime
) 

Now I am going to explain the use of After Trigger using Insert, Update, Delete statement with example

1. After Insert Trigger

   
    -- Create trigger on table Employee_Demo for Insert statement
   CREATE TRIGGER trgAfterInsert on Employee_Demo
   FOR INSERT
   AS declare @empid int, @empname varchar(55), @empsal decimal(10,2), @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_Demo_Audit(Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp)
   values (@empid,@empname,@empsal,@audit_action,getdate());
   PRINT 'AFTER INSERT trigger fired.'
   --Output will be 
   
   

   

   
    --Now try to insert data in Employee_Demo table
   insert into Employee_Demo(Emp_Name,Emp_Sal)values ('Shailu',1000);
   --Output will be 
   
   

   

   
    --now select data from both the tables to see trigger action
   select * from Employee_Demo
   select * from Employee_Demo_Audit
   --Output will be 
   
   

   
   Trigger have inserted the new record to Employee_Demo_Audit table for insert statement. In this way we can trace a insert activity on a table using trigger.

2. After Update Trigger

   
    -- Create trigger on table Employee_Demo for Update statement
   CREATE TRIGGER trgAfterUpdate ON dbo.Employee_Demo
   FOR UPDATE
   AS
   declare @empid int, @empname varchar(55), @empsal decimal(10,2), @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='Update Record --- After Update Trigger.';
   if update (Emp_Sal)
    set @audit_action='Update Record --- After Update Trigger.';
   insert intoEmployee_Demo_Audit(Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp)
   values (@empid,@empname,@empsal,@audit_action,getdate());
   PRINT 'AFTER UPDATE trigger fired.'
   --Output will be 
   
   

   

   
    --Now try to upadte data in Employee_Demo table
   update Employee_Demo set Emp_Name='Pawan' Where Emp_ID =6;
   --Output will be 
   
   

   

   
    --now select data from both the tables to see trigger action
   select * from Employee_Demo
   select * from Employee_Demo_Audit
   --Output will be 
   
   

   
   Trigger have inserted the new record to Employee_Demo_Audit table for update statement. In this way we can trace a update activity on a table using trigger.

3. After Delete Trigger

   
    -- Create trigger on table Employee_Demo for Delete statement
   CREATE TRIGGER trgAfterDelete ON dbo.Employee_Demo
   FOR DELETE
   AS
   declare @empid int, @empname varchar(55), @empsal decimal(10,2), @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;
   select @audit_action='Deleted -- After Delete Trigger.';
   insert into Employee_Demo_Audit (Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp)
   values (@empid,@empname,@empsal,@audit_action,getdate());
   PRINT 'AFTER DELETE TRIGGER fired.'
   --Output will be 
   
   

   

   
    --Now try to delete data in Employee_Demo table
   DELETE FROM Employee_Demo where emp_id = 5
   --Output will be 
   
   

   

   
    --now select data from both the tables to see trigger action
   select * from Employee_Demo
   select * from Employee_Demo_Audit
   --Output will be 
   
   

   
   Trigger have inserted the new record to Employee_Demo_Audit table for delete statement. In this way we can trace a delete activity on a table using trigger.

Now I am going to explain the use of Instead of Trigger using Insert, Update, Delete statement with example

1. Instead of Insert Trigger

   
    -- Create trigger on table Employee_Demo for Insert statement
   CREATE TRIGGER trgInsteadOfInsert ON dbo.Employee_Demo
   INSTEAD OF Insert
   AS
   declare @emp_id int, @emp_name varchar(55), @emp_sal decimal(10,2), @audit_action varchar(100);
   select @emp_id=i.Emp_ID from inserted i;
   select @emp_name=i.Emp_Name from inserted i;
   select @emp_sal=i.Emp_Sal from inserted i;
   SET @audit_action='Inserted Record -- Instead Of Insert Trigger.';
   BEGIN 
    BEGIN TRAN
    SET NOCOUNT ON
    if(@emp_sal>=1000)
    begin
    RAISERROR('Cannot Insert where salary < 1000',16,1); ROLLBACK; end
    else begin Insert into Employee_Demo (Emp_Name,Emp_Sal) values (@emp_name,@emp_sal); Insert into Employee_Demo_Audit(Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp) values(@@identity,@emp_name,@emp_sal,@audit_action,getdate());
    COMMIT;
    PRINT 'Record Inserted -- Instead Of Insert Trigger.'
   END
   --Output will be 
   
   

   

   
    --Now try to insert data in Employee_Demo table
   insert into Employee_Demo values ('Shailu',1300)
   insert into Employee_Demo values ('Shailu',900) -- It will raise error since we are checking salary >=1000
   --Outputs will be 
   
   

    

   
    --now select data from both the tables to see trigger action
   select * from Employee_Demo
   select * from Employee_Demo_Audit
   --Output will be 
   
   

   
   Trigger have inserted the new record to Employee_Demo_Audit table for insert statement. In this way we can apply business validation on the data to be inserted using Instead of trigger and can also trace a insert activity on a table.

2. Instead of Update Trigger

   
    -- Create trigger on table Employee_Demo for Update statement
   CREATE TRIGGER trgInsteadOfUpdate ON dbo.Employee_Demo
   INSTEAD OF Update
   AS
   declare @emp_id int, @emp_name varchar(55), @emp_sal decimal(10,2), @audit_action varchar(100);
   select @emp_id=i.Emp_ID from inserted i;
   select @emp_name=i.Emp_Name from inserted i;
   select @emp_sal=i.Emp_Sal from inserted i;
   BEGIN
    BEGIN TRAN
   if(@emp_sal>=1000)
    begin
    RAISERROR('Cannot Insert where salary < 1000',16,1); ROLLBACK; end
    else begin 
    insert into Employee_Demo_Audit(Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp) values(@emp_id,@emp_name,@emp_sal,@audit_action,getdate());
    COMMIT;
    PRINT 'Record Updated -- Instead Of Update Trigger.'; END
   --Output will be 
   
   

   

   
    --Now try to upadte data in Employee_Demo table
   update Employee_Demo set Emp_Sal = '1400' where emp_id = 6
   update Employee_Demo set Emp_Sal = '900' where emp_id = 6
   --Output will be 
   
   

    

   
    --now select data from both the tables to see trigger action
   select * from Employee_Demo
   select * from Employee_Demo_Audit
   --Output will be 
   
   

   
   Trigger have inserted the updated record to Employee_Demo_Audit table for update statement. In this way we can apply business validation on the data to be updated using Instead of trigger and can also trace a update activity on a table.

3. Instead of Delete Trigger

   
    -- Create trigger on table Employee_Demo for Delete statement
   CREATE TRIGGER trgAfterDelete ON dbo.Employee_Demo
   INSTEAD OF DELETE
   AS
   declare @empid int, @empname varchar(55), @empsal decimal(10,2), @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;
   BEGIN TRAN if(@empsal>1200) begin
    RAISERROR('Cannot delete where salary > 1200',16,1);
    ROLLBACK;
    end
    else begin
    delete from Employee_Demo where Emp_ID=@empid;
    COMMIT;
    insert into Employee_Demo_Audit(Emp_ID,Emp_Name,Emp_Sal,Audit_Action,Audit_Timestamp)
    values(@empid,@empname,@empsal,'Deleted -- Instead Of Delete Trigger.',getdate());
    PRINT 'Record Deleted -- Instead Of Delete Trigger.' end END
   --Output will be 
   
   

   

   
    --Now try to delete data in Employee_Demo table
   DELETE FROM Employee_Demo where emp_id = 1
   DELETE FROM Employee_Demo where emp_id = 3
   --Output will be 
   
   

    

   
    --now select data from both the tables to see trigger action
   select * from Employee_Demo
   select * from Employee_Demo_Audit
   --Output will be 
   
   

   
   Trigger have inserted the deleted record to Employee_Demo_Audit table for delete statement. In this way we can apply business validation on the data to be deleted using Instead of trigger and can also trace a delete activity on a table.

Now over to you:

"A blog is nothing without reader's feedback and comments. So please provide your valuable feedback so that i can make this blog better and If you like my work, you can appreciate by leaving your comments. Stay tuned and stay connected for more technical updates."