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:
- After Triggers (For Triggers)
- 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:
- AFTER INSERT Trigger.
- AFTER UPDATE Trigger.
- 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:-
CREATE TABLE Employee_Test_Audit ( Emp_ID int, Emp_name varchar(100), Emp_Sal decimal (10,2), Audit_Action varchar(100), Audit_Timestamp datetime )
Audit Table
Audit Tables are used to track transactions against a particular table or tables. They allow you to see an ongoing "log" (for lack of a better word)
create table SensitiveInformation
(
SensitiveNumber int not null,
SensitiveData varchar(100) not null
)
go
There may be users and/or applications that have access to insert, update, and delete out of that table. But due to the sensitive nature of that data, you may want to have a quick and easy way to track who is doing what on that table.
So you have an Audit Table to track what is being done on that table. Typically that'll include the basic Who, What, When.
An audit table could look like this:
create table SensitiveInformationAudit
(
SensitiveNumberNew int null,
SensitiveNumberOld int null,
SensitiveDataNew varchar(100) null,
SensitiveDataOld varchar(100) null,
Action varchar(50) not null,
AuditDate datetime not null,
LastUpdatedUser varchar(100) not null
)
go
Audit Tables are typically filled through the use of Database Triggers. In other words, when
X action happens on SensitiveInformation, insert the details of it in SensitiveInformationAudit.(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.' GOThe
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.' GOThe 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=6This 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.' GOIn 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 12:52:13.867All the triggers can be enabled/disabled on the table using the statement
ALTER TABLE Employee_Test {ENABLE|DISBALE} TRIGGER ALLSpecific Triggers can be enabled or disabled as:
ALTER TABLE Employee_Test DISABLE TRIGGER trgAfterDeleteThis disables the After Delete Trigger named
trgAfterDelete on the specified table.(ii) 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 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:
- INSTEAD OF INSERT Trigger.
- INSTEAD OF UPDATE Trigger.
- INSTEAD OF DELETE Trigger.
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 GOThis 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=4This 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 > 1200And this record will not be deleted.
In a similar way, you can code Instead of Insert and Instead Of Update triggers on your tables.
