Database : Different Types of SQL Server Triggers

iggers are database object. Basically these are special type of stored procedure that are automatically fired/executed when a DDL or DML command statement related with the trigger is executed. Triggers are used to assess/evaluate data before or after data modification using DDL and DML statements. These are also used to preserve data integrity, to control server operations, to audit a server and to implement business logic or business rule.

Types of Triggers

In Sql Server we can create four types of triggers Data Definition Language (DDL) triggers, Data Manipulation Language (DML) triggers, CLR triggers and Logon triggers.

1. DDL Triggers

   In SQL Server we can create triggers on DDL statements (like CREATE, ALTER, and DROP) and certain system defined stored procedures that perform DDL-like operations.
   Example : If you are going to execute the CREATE LOGIN statement or the sp_addlogin stored procedure to create login user, then both these can execute/fire a DDL trigger that you can create on CREATE_LOGIN event of Sql Server.
   We can use only FOR/AFTER clause in DDL triggers not INSTEAD OF clause means we can make only After Trigger on DDL statements.
   DDL trigger can be used to observe and control actions performed on the server, and to audit these operations. DDL triggers can be used to manage administrator tasks such as auditing and regulating database operations.

2. DML Triggers

   In SQL Server we can create triggers on DML statements (like INSERT, UPDATE, and DELETE) and stored procedures that perform DML-like operations. DML Triggers are of two types

   1. After Trigger (using FOR/AFTER CLAUSE)

      This type of trigger fires after SQL Server finish the execution of the action successfully that fired it.
      Example : If you insert record/row in a table then the trigger related/associated with the insert event on this table will fire only after the row passes all the constraints, like as primary key constraint, and some rules. If the record/row insertion fails, SQL Server will not fire the After Trigger.

   2. Instead of Trigger (using INSTEAD OF CLAUSE)

      This type of trigger fires before SQL Server starts the execution of the action that fired it. This is differ from the AFTER trigger, which fires after the action that caused it to fire. We can have an INSTEAD OF insert/update/delete trigger on a table that successfully executed but does not include the actual insert/update/delete to the table.
      Example : If you insert record/row in a table then the trigger related/associated with the insert event on this table will fire before the row passes all the constraints, such as primary key constraint and some rules. If the record/row insertion fails, SQL Server will fire the Instead of Trigger.

3. CLR Triggers

   CLR triggers are special type of triggers that based on the CLR (Common Language Runtime) in .net framework. CLR integration of triggers has been introduced with SQL Server 2008 and allows for triggers to be coded in one of .NET languages like C#, Visual Basic and F#.
   We coded the objects(like trigger) in the CLR that have heavy computations or need references to objects outside the SQL Server. We can write code for both DDL and DML triggers, using a supported CLR language like C#, Visual basic and F#. I will discuss CLR trigger later.

4. Logon Triggers

   Logon triggers are special type of trigger that fire when LOGON event of Sql Server is raised. This event is raised when a user session is being established with Sql Server that is made after the authentication phase finishes, but before the user session is actually established. Hence, all messages that we define in the trigger such as error messages, will be redirected to the SQL Server error log. Logon triggers do not fire if authentication fails. We can use these triggers to audit and control server sessions, such as to track login activity or limit the number of sessions for a specific login.
   Synatx for Logon Trigger

   
   CREATE TRIGGER trigger_name
   ON ALL SERVER
   [WITH ENCRYPTION]
   {FOR|AFTER} LOGON
   AS
   sql_statement [1...n ] 
   
   

Syntax for Trigger

CREATE TRIGGER trigger_name
ON {table|view} 
[WITH ENCRYPTION|EXECUTE AS] 
{FOR|AFTER|INSTEAD OF} {[CREATE|ALTER|DROP|INSERT|UPDATE|DELETE ]} 
[NOT FOR REPLICATION] 
AS 
sql_statement [1...n ] 

1. trigger_name

   This is the name of the trigger. It should conform to the rules for identifiers in Sql Server.

2. table|view

   This is the table/view on which the trigger is to be created.

3. ENCRYPTION

   This option is optional. If this option is specified, original text of the CREATE TRIGGER statement will be encrypted.

4. EXECUTE AS

   This option is optional. This option specifies, the security context under which the trigger is executed.

5. FOR/AFTER

   FOR/AFTER specifies that the trigger is After Trigger. AFTER is the default, if FOR is the only keyword specified.AFTER triggers cannot be defined on views.

6. INSTEAD OF

   INSTEAD OF specifies that the trigger is Instead Of Trigger.

7. CREATE|ALTER|DROP|INSERT|UPDATE|DELETE

   These keywords specify on which action the trigger should be fired. One of these keywords or any combination of these keywords in any order can be used.

8. NOT FOR REPLICATION

   Indicates that the trigger should not be executed when a replication process modifies the table involved in the trigger.

9. AS

   After this we specifies the actions and condition that the trigger perform.

10. sql_statement

    These are the trigger conditions and actions. The trigger actions specified in the T-SQL statements.

Note

1. The name of a trigger should follow the rules for identifiers.
2. DML trigger can be composed by any T-SQL statements, except CREATE DATABASE, ALTER DATABASE, DROP DATABASE, LOAD DATABASE, LOAD LOG, RECONFIGURE, RESTORE DATABASE, and RESTORE LOG statements.
3. You cannot create triggers against system tables or dynamic management views. Moreover, the TRUNCATE TABLE statement does not fire a trigger because this operation does not log individual row deletions.
4. If you use the DATABASE option, the scope of your DDL trigger will be the current database. If you use the ALL SERVER option, the scope of your DDL triggers to the current server.
5. AFTER triggers cannot be defined on views.
6. AFTER is the default, if FOR is the only keyword specified.