Changing the default behavior of DML statements (INSERT/UPDATE/DELETE)

Generally we are using DML statements like Insert for inserting data into database table likewise Update for updating existing data in database table, but it is also possible to change the default behavior INSERT/UPDATE/DELETE (DML) statements for example while inserting data using INSERT statement, the new record may applies as an update to the table or even applies as a delete to the table.

You can implement the changing behavior with INSTEAD OF TRIGGER. Let’s do some practical to achieve the changing behavior of DML statements.

use testDemo

go

if object_id('ChangingDMLBehavior') is not null

      drop table ChangingDMLBehavior

go

     

We were checking the existence of 'ChangingDMLBehavior' table object.

Now lets create a table 'ChangingDMLBehavior'

create table ChangingDMLBehavior

(id int not null identity(1,1),

name varchar(10))

Command(s) completed successfully.

Table is now ready; it’s time to write an INSTEAD OF TRIGGER to stimulate the changing behavior of INSERT statement.

 create trigger trg_DMLBehavior on ChangingDMLBehavior

instead of insert

as

begin

      select * from inserted

      select * from deleted

      return

end

go

Command(s) completed successfully.

Now since Trigger is in place, now it’s time to try out INSERT statement on 'ChangingDMLBehavior' and check out the behavior of INSERT Statement.

 insert into ChangingDMLBehavior(name) values('Binny')

Now querying the table to find the inserted records, to our shock we found no records.

select * from ChangingDMLBehavior

id          name

----------- ----------

(0 row(s) affected)

This is because a record is inserted to table 'ChangingDMLBehavior' with INSERT command, but nothing happens to the table due to the instead of trigger. This is because the trigger does not tell SQL Server to perform any data manipulation operation.

Let’s do some modification of INSTEAD OF TRIGGER to allow INSERT OPTION on table.

alter trigger trg_DMLBehavior on ChangingDMLBehavior

instead of insert

as

begin

      set nocount on

      insert into ChangingDMLBehavior(name)

            select name from inserted

      return

end

go

Command(s) completed successfully.

Now we can fire INSERT statement on 'ChangingDMLBehavior' table and can expect insertion of record to the table. Lets insert one record into 'ChangingDMLBehavior'.

insert into ChangingDMLBehavior(name) values('Prachi')

(1 row(s) affected)

Querying the table we get

select * from ChangingDMLBehavior

id          name

----------- ----------

1           Prachi

(1 row(s) affected)

Instead of triggers are useful when you want some setting kind of table in database to hold the defined values.

When you are in love with your code you will stop dreaming code with closed eyes because reality better than your dreams

Post Reference: Vikram Aristocratic Elfin Share