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
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.
Let’s create After triggers. First of all, let’s create a table and insert some sample data. Then, on this table, I will be attaching several triggers.
AFTER TRIGGERS can be classified further into three types as:
(a) AFTER INSERT Trigger.
(b) AFTER UPDATE Trigger.
(c) AFTER DELETE Trigger.
Let’s create After triggers. First of all, let’s create a table and insert some sample data. Then, on this table, I will be attaching several triggers.
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);
I will be
creating an AFTER INSERT TRIGGER which will insert the rows inserted into the
table into another audit table. The main purpose of this audit table is to
record the changes in the main table. This can be thought of as a generic audit
trigger.
Now, create the audit table as:-
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) AFTRE 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.
To see the newly created trigger in action, lets insert a row into the main table as :
insert into Employee_Test values('Chris',1500);
Now, a record
has been inserted into the Employee_Test table. The AFTER INSERT trigger
attached to this table has inserted the record into the Employee_Test_Audit
as:-
6 Chris 1500.00
Inserted Record -- After Insert Trigger. 2008-04-26
12:00:55.700
(b) 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 theupdate(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.
Let’s update a record column and see what happens.
Let’s update a record column and see what happens.
update Employee_Test set Emp_Sal=1550 where Emp_ID=6
This inserts
the row into the audit table as:-
6 Chris 1550.00
Updated Record -- After Update Trigger. 2008-04-26 12:38:11.843
(c) 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.
Let’s fire a delete on the main table.
Let’s fire a delete on the main table.
A record has been inserted into the audit table as:-
6 Chris 1550.00
Deleted -- After Delete Trigger.
2008-04-26 12:52:13.867
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 anyonr 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,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.
Now, let’s try to delete a record with the Emp_Sal >1200 as:-
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.
In a similar way,
you can code Instead of Insert and Instead Of Update triggers on your tables.
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