Triggers in SQL Server

Triggers

Creates a DML, DDL, or logon trigger. A trigger is a special kind of stored procedure that automatically executes when an event occurs in the database server. DML triggers execute when a user tries to modify data through a data manipulation language (DML) event. DML events are INSERT, UPDATE, or DELETE statements on a table or view.

DML Triggers

DML triggers are frequently used for enforcing business rules and data integrity. SQL Server provides declarative referential integrity (DRI) through the ALTER TABLE and CREATE TABLE statements. However, DRI does not provide cross-database referential integrity. Referential integrity refers to the rules about the relationships between the primary and foreign keys of tables. To enforce referential integrity, use the PRIMARY KEY and FOREIGN KEY constraints in ALTER TABLE and CREATE TABLE. If constraints exist on the trigger table, they are checked after the INSTEAD OF trigger execution and before the AFTER trigger execution. If the constraints are violated, the INSTEAD OF trigger actions are rolled back and the AFTER trigger is not fired.

DDL Triggers

In your workplace does more than one person has access to databases that are vital to the smooth operation of your organisation? Do these users have rights to alter the database structure by using Data Definition Language (DDL) statements such as CREATE TABLE..., DROP TABLE etc?

If the answer to the above questions are yes then are mechanisms in place to help monitor/audit who is making what changes to the database? Who deleted that table? Who changed that columns data type? What code was in the previous version of that procedure that now isn't working? This type of auditing is being asked for more and more as organisations become more and more dependent on their databases for helping in all aspects of day to day work.

DDL Triggers (introduced in SQL Server 2005) provide you with the capability of auditing the creation, deletion and modification of database objects as well as other capabilities such as checking that DDL code conforms to your business rules before executing it.

How Triggers work

A Trigger is a block of T-SQL code that is executed or 'triggered' as a result of another statement that is sent to the database. Before SQL Server 2005 a trigger could be 'triggered' by either INSERT, UPDATE or DELETE (Data Manipulation Language - DML) statements. SQL Server 2005 introduced DML Triggers that can be set to fire on your chosen DDL events such as CREATE_TABLE, ALTER_TABLE, DROP_TABLE, ALTER_DATABASE, CREATE_LOGIN etc.

DDL Triggers can be set with either a Server scope or database scope. Triggers created with Server scope must target server DDL events such as CREATE_DATABASE or CREATE_LOGIN whilst triggers created with database scope must target database level events such as CREATE_TABLE or ALTER_PROC. See the full list of SQL Server DDL Trigger Events (including their scope).

DDL triggers can only fire after the DDL statement has occurred. This is different from DML triggers which can fire before the triggering statement.

Syntax of a DDL trigger

CREATE TRIGGER [TriggerName]
ON [Scope (Server|Database)]
FOR [EventName...],
AS
-- code for your trigger response here

The EventData function

If you want to audit changes to your database schemas you need to be able to access the triggering events in your DDL trigger so that you can record what changes are being made. To access the triggering event we can use the EventData function in our DDL trigger. The EventData function returns an xml value.

The EventData xml value includes the triggering SQL statement, the event time, the type of event and depending on what type of event was called, extra information such as the database name. The following example shows how EventData can be used to record all statements that changed the table or stored proc schemas into a table called DDLAudit.

CREATE TRIGGER AuditProcChanges
ON DATABASE
FOR CREATE_PROC, ALTER_PROC, DROP_PROC, CREATE_TABLE, ALTER_TABLE, DROP_TABLE
AS

DECLARE @ed XML
SET @ed = EVENTDATA()

INSERT INTO DDLAudit (PostTime, DatabaseName, Event, ObjectName, TSQL, Login)
VALUES
(
GetDate(),
@ed.value('(/EVENT_INSTANCE/DatabaseName)[1]', 'varchar(256)'),
@ed.value('(/EVENT_INSTANCE/EventType)[1]', nvarchar(100)'),
@ed.value('(/EVENT_INSTANCE/ObjectName)[1]', 'varchar(256)'),
@ed.value('(/EVENT_INSTANCE/TSQLCommand)[1]', 'nvarchar(2000)'),
@ed.value('(/EVENT_INSTANCE/LoginName)[1]', 'varchar(256)')
)

The EventData function returns an xml value and is assigned to a variable called @ed which is of an xml data type. The xquery function value(Xquery, sqltype) returns the specified values from the xml variable. For more information on EventData see MSDN.

This DDLAudit table could reside in the individual database or you could create a seperate ApplicationAudit database and use a 3 part name to record the audit in this ApplicationAudit database, i.e. INSERT INTO ApplicationAudit.dbo.DDLAudit ....

Covering all databases and events

If you wanted to audit the DDL events for all your databases you would need to create this trigger in each database. The above trigger only monitors DDL events that affect Tables and Stored Procedures. You can use the handy event name of DDL_DATABASE_LEVEL_EVENTS to make sure your trigger covers all DDL events that have database scope as follows:

CREATE TRIGGER AuditDBScopeDDLChanges
ON DATABASE
FOR DDL_DATABASE_LEVEL_EVENTS
AS
-- trigger code here...

Preventing DDL actions using triggers

Sometimes you may want to prevent the alteration of a schema, because the triggering statement and trigger are joined in one transaction we can call ROLLBACK in our trigger to rollback the DDL statement that caused the trigger to fire:

CREATE TRIGGER PreventDropTable
ON DATABASE
FOR DROP_TABLE
AS
PRINT 'Tables cannot be dropped'
ROLLBACK

What happens if you then want to drop a table in the database with the above trigger? You can disable the trigger, drop the table and then re-enable the trigger:

DISABLE TRIGGER PreventDropTable
ON DATABASE
GO
DROP TABLE MyTable
GO
ENABLE TRIGGER PreventDropTable
ON DATABASE
GO

You could also if you wished extend use this method of schema change prevention for server scope events to prevent the dropping of databases:

CREATE TRIGGER PreventDropDatabaseServerWide
ON ALL SERVER
FOR DROP_DATABASE
AS
PRINT 'Cannot drop tables, DDL Trigger will rollback'
ROLLBACK

Finding DDL triggers using system tables

To view the DDL triggers in your databases of database scope you can query the sys.triggers table. To view the DDL triggers with server scope you need to query the sys.server_triggers table in the master database.

Conclusion

When designing your DDL trigger you will probably be performing one of more of the following actions:

• Recording changes made to the database schema
• Stopping certain types of changes being made to the database schema
• Fire another action in the database in response to the schema change

We have seen how DDL triggers can be used to a) audit and b) control schema changes using the EventData function and the ROLLBACK command respectively.

Logon Triggers

Logon triggers fire in response to the LOGON event. This event is raised when a user sessions is being established. For more information.

Multiple Triggers

SQL Server allows for multiple triggers to be created for each DML, DDL, or LOGON event. For example, if CREATE TRIGGER FOR UPDATE is executed for a table that already has an UPDATE trigger, an additional update trigger is created. In earlier versions of SQL Server, only one trigger for each INSERT, UPDATE, or DELETE data modification event is allowed for each table.

Recursive Triggers

SQL Server also allows for recursive invocation of triggers when the RECURSIVE_TRIGGERS setting is enabled using ALTER DATABASE.

Nested Triggers

Triggers can be nested to a maximum of 32 levels. If a trigger changes a table on which there is another trigger, the second trigger is activated and can then call a third trigger, and so on. If any trigger in the chain sets off an infinite loop, the nesting level is exceeded and the trigger is canceled. To disable nested triggers, set the nested triggers option of sp_configure to 0 (off). The default configuration allows for nested triggers. If nested triggers is off, recursive triggers is also disabled, regardless of the RECURSIVE_TRIGGERS setting set by using ALTER DATABASE.