Implementing Triggers in SQL Server 2000

Triggers are special types of Stored Procedures that are defined to execute automatically in place of or after data modifications. They can be executed automatically on the INSERT, DELETE and UPDATE triggering actions.

There are two different types of triggers in Microsoft SQL Server 2000. They are INSTEAD OF triggers and AFTER triggers. These triggers differ from each other in terms of their purpose and when they are fired. In this article we shall discuss each type of trigger.

First of all, let's create a sample database with some tables and insert some sample data in those tables using the script below:



Create Database KDMNN
GO
USE KDMNN
GO
CREATE TABLE [dbo].[User_Details] (
[UserID] [int] NULL ,
[FName] [varchar] (50) NOT NULL ,
[MName] [varchar] (50) NULL ,
[LName] [varchar] (50) NOT NULL ,
[Email] [varchar] (50) NOT NULL
) ON [PRIMARY]
GO

CREATE TABLE [dbo].[User_Master] (
[UserID] [int] IDENTITY (1, 1) NOT NULL ,
[UserName] [varchar] (50) NULL ,
[Password] [varchar] (50) NULL
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[User_Master] WITH NOCHECK ADD CONSTRAINT [PK_User_Master] PRIMARY KEY CLUSTERED
(
[UserID]
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[User_Details] ADD CONSTRAINT [FK_User_Details_User_Master] FOREIGN KEY (
[UserID]
) REFERENCES [dbo].[User_Master] ( [UserID] )
GO

INSERT INTO USER_MASTER(USERNAME, PASSWORD)
SELECT 'Navneeth','Navneeth' UNION
SELECT 'Amol','Amol' UNION
SELECT 'Anil','Anil' UNION
SELECT 'Murthy','Murthy'

INSERT INTO USER_DETAILS(USERID, FNAME, LNAME, EMAIL)
SELECT 1,'Navneeth','Naik','navneeth@kdmnn.com' UNION
SELECT 2,'Amol','Kulkarni','amol@kdmnn.com' UNION
SELECT 3,'Anil','Bahirat','anil@kdmnn.com' UNION
SELECT 4,'Murthy','Belluri','murthy@kdmnn.com'

AFTER Triggers
The type of trigger that gets executed automatically after the statement that triggered it completes is called an AFTER trigger. An AFTER trigger is a trigger that gets executed automatically before the transaction is committed or rolled back.

Using the below script, first we shall create a trigger on the table USER_MASTER for the INSERT event of the table.

USE KDMNN
Go

CREATE TRIGGER trgInsert ON User_Master
FOR INSERT AS
Print ('AFTER Trigger [trgInsert] – Trigger executed !!')
GO


BEGIN TRANSACTION
DECLARE @ERR INT
INSERT INTO USER_MASTER(USERNAME, PASSWORD) VALUES('Damerla','Damerla')
SET @ERR = @@Error
IF @ERR > 0
BEGIN
ROLLBACK TRANSACTION
PRINT 'ROLLBACK TRANSACTION'
END

ELSE
BEGIN COMMIT TRANSACTION
PRINT 'COMMIT TRANSACTION'
END

Output
AFTER Trigger [trgInsert] – Trigger executed !!
(1 row(s) affected)
ROLLBACK TRANSACTION

By looking at the output, we can conclude that before the transaction is rolled back or committed, the AFTER trigger gets executed automatically.

A table can have several AFTER triggers for each of the three triggering actions i.e., INSERT, DELETE and UPDATE. Using the below script, we shall create two triggers on the table User_Master for the INSERT triggering action.

CREATE TRIGGER trgInsert2
ON User_Master
FOR INSERT
AS
BEGIN
Print ('AFTER Trigger [trgInsert2] – Trigger executed !!')
END
GO

CREATE TRIGGER trgInsert3
ON User_Master
FOR INSERT
AS
BEGIN
Print ('AFTER Trigger [trgInsert3] – Trigger executed !!')
END
GO

BEGIN TRANSACTION
DECLARE @ERR INT
INSERT INTO USER_MASTER(USERNAME, PASSWORD)
VALUES('Damerla','Damerla')
SET @ERR = @@Error
IF @ERR > 0
BEGIN
ROLLBACK TRANSACTION
PRINT 'ROLLBACK TRANSACTION'
END
ELSE
BEGIN
COMMIT TRANSACTION
PRINT 'COMMIT TRANSACTION'
END

Output
AFTER Trigger [trgInsert] – Trigger executed !!
AFTER Trigger [trgInsert2] – Trigger executed !!
AFTER Trigger [trgInsert3] – Trigger executed !!
(1 row(s) affected)
ROLLBACK TRANSACTION

From the output we can conclude that when the user tries to insert data in the table USER_MASTER, three triggers are executed automatically. That is, you can write several AFTER triggers on one table for each of the three triggering actions.

Similarly, we can write several AFTER triggers on DELETE and UPDATE triggering actions.

Note: If a table has multiple AFTER triggers, then you can specify which trigger should be executed first and which trigger should be executed last using the stored procedure sp_settriggerorder. All the other triggers are in an undefined order which you cannot control.

An AFTER trigger can be created only on tables, not on views.

Using the script below, first we shall create a simple view [vwUserMaster] which will fetch the Username and Password from the table USER_MASTER.


Create View vwUserMaster as SELECT USERNAME, PASSWORD FROM USER_MASTER
GO

CREATE TRIGGER trgOnViewON vwUserMaster
FOR INSERT
AS
BEGIN
Print ('AFTER Trigger [trgOnView] – vwUserMaster !!')
END
GO


Output

Server: Msg 208, Level 16, State 4, Procedure trgOnView, Line 2
Invalid object name 'vwUserMaster'.

From the Output we can conclude that we cannot create an AFTER trigger on views.
Like stored procedures and views, triggers can also be encrypted.

The trigger definition is then stored in an unreadable form. Once encrypted, the definition of the trigger cannot be decrypted and cannot be viewed by anyone, including the owner of the trigger or the system administrator.

CREATE TRIGGER trgEncrypted
ON User_Master WITH ENCRYPTION
FOR INSERT
AS
BEGIN
Print ('AFTER Trigger [trgEncrypted] Encrypted – Trigger executed !!')
END
GO

SELECT sysobjects.name AS [Trigger Name],
SUBSTRING(syscomments.text, 0, 26) AS [Trigger Definition],
OBJECT_NAME(sysobjects.parent_obj) AS [Table Name],
syscomments.encrypted AS [IsEncrpted]

FROM sysobjects INNER JOIN syscomments ON sysobjects.id = syscomments.id
WHERE
(sysobjects.xtype = 'TR')

Output
Trigger Name Trigger Definition Table Name IsEncrpted

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

trgInsert CREATE TRIGGER trgInsert User_Master 0
trgInsert1 CREATE TRIGGER trgInsert1 User_Master 0
trgInsert2 CREATE TRIGGER trgInsert2 User_Master 0
trgEncrypted ??????????????? User_Master 1

Since the trigger trgEncrypted is created with the option WITH ENCRYPTION, the trigger definition is hidden and there is no way that one can easily decrypt the trigger code.

We all know that the DML statements change or modify data. Sometimes it becomes necessary for the triggers to have the access to the changes being caused by the DML statements. SQL Server 2000 provides four different ways to determine the affects of the DML statements. The INSERTED and DELETED tables, popularly known as MAGIC TABLES, and update () and columns_updated() functions can be used to determine the changes being caused by the DML statements.

The below table depicts the contents of the INSERTED and DELETED tables for three different table Events

EVENT
INSERTED
DELETED

Insert
Contains the inserted rows
Empty

Delete
Empty
Contains the rows to be deleted

Update
Contains the rows after update
Contains the rows before update

Note that the Magic Table does not contain the information about the columns of the data-type text, ntext, or image. Attempting to access these columns will cause an error.

The update() function is used to find whether a particular column has been updated or not. This function is generally used for data checks.


CREATE TRIGGER trgUddate ON User_Details
FOR UPDATE
AS
If UPDATE(FName)
BEGIN
PRINT('AFTER Trigger [trgUddate] - Executed - First Name has been updated')
ROLLBACK TRANSACTION
END
else If UPDATE(LName)

BEGIN
PRINT('AFTER Trigger [trgUddate] - Executed - Last Name has been updated')
ROLLBACK TRANSACTION
END
else If UPDATE(MName)
BEGIN
PRINT('AFTER Trigger [trgUddate] - Executed - MName Name has been updated')
ROLLBACK TRANSACTION
END
else If UPDATE(Email)
BEGIN
PRINT('AFTER Trigger [trgUddate] - Executed - Email has been updated')
ROLLBACK TRANSACTION
END

GO


UPDATE User_Details
SET MName = 'Diwaker'
WHERE UserID = 1

Output

AFTER Trigger [trgUddate] - Executed - MName Name has been updated

Depending upon the column updated, a message will be displayed. With this feature we can determine which column in the table has been updated, and then proceed with the business rules to be implemented further.

Columns_Update() function returns a varbinary data type representation of the columns updated. This function return a hexadecimal values from which we can determine which columns in the table have been updated.


INSTEAD OF Triggers

A trigger which gets executed automatically in place of triggering actions i.e., INSERT, DELETE and UPDATE is called an INSTEAD OF trigger.

INSTEAD OF triggers gets executed automatically before the Primary Key and the Foreign Key constraints are checked, whereas the traditional AFTER triggers gets executed automatically after these constraints are checked.


CREATE TRIGGER trgAfterInsert On User_Details
FOR INSERT
AS
BEGIN
Print ('AFTER Trigger [trgAfterInsert] – Trigger executed !!')
END


INSERT INTO USER_DETAILS(USERID, FNAME,LNAME, MNAME, EMAIL)
VALUES(100, 'FName','LName','MName','test@test.com')

Server: Msg 547, Level 16, State 1, Line 1

INSERT statement conflicted with COLUMN FOREIGN KEY constraint 'FK_User_Details_User_Master'. The conflict occurred in database 'KDMNN', table 'User_Master', column 'UserID'.

The statement has been terminated.

UserID 100 does not exist in the User_Master table, so the Foreign Key constraint has been checked and an error message is displayed. What we can conclude is: AFTER triggers gets executed automatically after the PK and FK constraints.

Create Trigger trgInsteadInsert
On User_Details
INSTEAD OF INSERT
AS
BEGIN
Print ('INSTEAD OF Trigger [trgInsteadInsert] – Trigger executed !!')
END

INSERT INTO USER_DETAILS(USERID, FNAME,LNAME, MNAME, EMAIL)
VALUES(100, 'FName','LName','MName','test@test.com')

Output
INSTEAD OF Trigger [trgInsteadInsert] – Trigger executed !!
(1 row(s) affected)

Even if the UserID 100 does not exists in the User_Master table, the trigger gets gets executed automatically.

DROP Trigger trgInsteadInsert
Unlike AFTER triggers, INSTEAD OF triggers can be created on views.

Create trigger trgOnView
on vwUserMaster
INSTEAD OF INSERT
AS
begin
Print ('INSTEAD OF Trigger [trgOnView] – vwUserMaster !!!')
End

INSERT INTO VWUSERMASTER(USERNAME, PASSWORD)
VALUES('Damerla','Venkat')

Output
INSTEAD OF Trigger [trgOnView] – vwUserMaster !!
(1 row(s) affected)

So whenever a user tries to insert data into the view vwUserMaster, the INSTEAD OF trigger trgOnView will automatically be executed.

In SQL SERVER 2000, views can be used to INSERT/DELETE and UPDATE the data in the multiple tables, and this can be achieved using INSTEAD OF triggers.

CREATE VIEW vwUser
AS
SELECT
[User_Master].[Username],
[User_Master].[Password],
[User_Details].[FName],
[User_Details].[MName],
[User_Details].[LName],
[User_Details].[Email]
FROM
[User_Master], [User_Details]
WHERE
[User_Master].[UserID]=[User_Details].[UserID]

CREATE TRIGGER tgrInsertData
ON vwUser
INSTEAD OF INSERT
AS
BEGIN
Declare @UserName varchar(50)
Declare @Password varchar(50)
Declare @FName varchar(50)
Declare @MName varchar(50)
Declare @LName varchar(50)
Declare @Email varchar(50)

SELECT
@UserName = UserName,
@Password = Password,
@FName = FName,
@MName = MName,
@LName = LName,
@Email = Email
FROM INSERTED

INSERT INTO User_Master(UserName, Password)
VALUES(@UserName, @Password)

INSERT INTO User_Details(UserID,FName,LName,MName,Email) VALUES(@@Identity, @FName, @LName, @MName, @Email)

END

INSERT INTO vwUser(UserName,Password,FName,LName,MName,Email)
VALUES ('Dhananjay','Dhananjay','Dhananjay','Nagesh',NULL,
'Dhananjay@kdmnn.com'

Output

AFTER Trigger [trgInsert] – Trigger executed !!
AFTER Trigger [trgInsert2] – Trigger executed !!
AFTER Trigger [trgInsert3] – Trigger executed !!
AFTER Trigger [trgEncrypted] Encrypted – Trigger executed !!
(1 row(s) affected)
AFTER Trigger [trgAfterInsert] – Trigger executed !!
(1 row(s) affected)

Then check the data in the following tables User_Master and User_Details. The new row gets inserted in both the tables.

A view or table can have only one INSTEAD OF trigger for each INSERT, UPDATE and DELETE events.

We have seen that you can create any number of AFTER triggers on the table for the same event, whereas you cannot do that with INSTEAD OF triggers.

CREATE TRIGGER trgInsteadOfTrigger1
ON vwUserMaster
INSTEAD OF UPDATE
AS
BEGIN
Print ('INSTEAD OF Trigger [trgInsteadOfTrigger1] – Trigger executed !!')
END

CREATE TRIGGER trgInsteadOfTrigger2
ON vwUserMaster
INSTEAD OF UPDATE
AS
BEGIN
Print ('INSTEAD OF Trigger [trgInsteadOfTrigger2] – Trigger executed !!')
END

Output
Server: Msg 2111, Level 16, State 1, Procedure trgInsteadOfTrigger2, Line 6

Cannot CREATE trigger 'trgInsteadOfTrigger2' for view 'vwUserMaster' because an INSTEAD OF UPDATE trigger already exists.

From the output, it is clear that you cannot create two INSTEAD OF triggers on the view/ table for the same event.

Note: An important point to be noted is that INSTEAD OF DELETE and INSTEAD OF UPDATE triggers cannot be defined on tables that have corresponding ON DELETE or ON UPDATE cascading referential integrity defined.

At last, how would you know what are the triggers associated with the table and what type of the trigger it is? Whether AFTER or INSTEAD OF?

The solution for this question is sp_helptrigger. This stored procedure gives all the information about the triggers such as Event on which the trigger gets executed, the type of the trigger, etc.

Sp_helptrigger User_Master

Output
trigger_name trigger_owner isupdate isdelete isinsert isafter isinsteadof
trgInsert dbo 0 0 1 1 0
trgInsert2 dbo 0 0 1 1 0
trgInsert3 dbo 0 0 1 1 0
trgEncrypted dbo 0 0 1 1 0

Triggers can be used in the following scenarios, such as if the database is de-normalized and requires an automated way to update redundant data contained in multiple tables, or if customized messages and complex error handling are required, or if a value in one table must be validated against a non-identical value in another table.

Triggers are a powerful tool that can be used to enforce the business rules automatically when the data is modified. Triggers can also be used to maintain the data integrity. But they are not to maintain data integrity. Triggers should be used to maintain the data integrity only if you are unable to enforce the data integrity using CONSTRAINTS, RULES and DEFAULTS. Triggers cannot be created on the temporary tables.