Triggers

What is a Trigger?

A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or UPDATE) occurs. Triggers are stored in and managed by the DBMS. Triggers are used to maintain the referential integrity of data by changing the data in a systematic fashion. A trigger cannot be called or executed; DBMS automatically fires the trigger as a result of a data modification to the associated table. Triggers can be considered to be similar to stored procedures in that both consist of procedural logic that is stored at the database level. Stored procedures, however, are not event-driven and are not attached to a specific table as triggers are. Stored procedures are explicitly executed by invoking a CALL to the procedure while triggers are implicitly executed. In addition, triggers can also execute stored procedures.

Types of Triggers

There are three action query types that you use in SQL which are INSERT, UPDATE and DELETE.

DML Triggers:

1.       AFTER

These triggers run after an insert, update or delete on a table. They are not supported for views. 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:-

CREATE TABLE Employee_Test_Audit

(

Emp_ID int,

Emp_name varchar (100),

Emp_Sal decimal (10, 2),

Audit_Action varchar (100),

Audit_Timestamp datetime

)

AFTER TRIGGERS can be classified further into three types as:

(a)    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.

To see the newly created trigger in action, let’s 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 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.

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. 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

This disables the After Delete Trigger named trgAfterDelete on the specified table.

2.      INSTEAD OF

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. 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.

DDL Trigger

This type of trigger is fired against Drop Table, Create Table, Alter Table or Login events. DDL Triggers are always After Triggers.

Queries of Trigger

1.       Create trigger

CREATE TRIGGER after_insert on Employee

AFTER INSERT

AS

       declare @emp_id int;

       declare @f_name varchar(300);

       declare @sal varchar(300);

       select @emp_id = i.Employee_id from inserted i;

       select @f_name = i.First_name from inserted i;

       select @sal = i.Salary from inserted i;

       insert into Employee_test_Audit values(@emp_id, @f_name, @sal, 'After insert trigger fired', CURRENT_TIMESTAMP);

       PRINT 'AFTER INSERT trigger fired.'

GO

2.       Alter trigger                This changes the trigger definition.

ALTER TRIGGER after_insert on Employee

3.       RENAME trigger

sp_rename 'after_insert', 'after_insert_trigger';

4.       Drop trigger

DROP TRIGGER after_insert

5.       Enable trigger

ENABLE TRIGGER after_insert ON Employee

6.       Disable trigger

DISABLE TRIGGER after_insert ON Employee

7.       If Exists

If you want to check weather trigger exist before a drop, then use:

SELECT * FROM sys.triggers WHERE name = 'after_insert'

 SQL Trigger Interview Questions with Answers:

22) What is the difference between InsteadOf and After/For trigger?

InsteadOf trigger will be fired on behalf of DML statements whereas After trigger will be fired after either DDL or DML statements.