Friday, August 26, 2005

Basics of DDL Triggers in SQL Server 2005

This article would explain in detail about the new feature "DDL triggers" in SQL Server 2005. I have explained it with an example whose complete code snippet is also attached below.

Till SQL Server 2000 one can create DML triggers only. i.e., triggers which would fire on Insert, Update or Delete of a table or a view.

In SQL Server 2005 (Yukon) we can create triggers for DDL (Data definition Language) statements. i.e., we can write triggers which would react to CREATE, ALTER & DROP statements as well.

Syntax for creating DDL statements:

Create Trigger trigger_name
ON { ALL SERVER DATABASE }
[ WITH ENCRYPTION ]
{ FOR AFTER } { event_type event_group } [ ,...n ]
AS { sql_statement [ ...n ]
EXTERNAL NAME <> }
[ ; ]
::=
assembly_name.class_name[.method_name]

Major points which I thought might interest you are:

i) The ON clause in a DDL trigger refers to either the scope of the whole database server (ALL SERVER) or the current database (DATABASE).

ii) Event_type argument is used to define the event for which the trigger would fire.

iii) Inside a trigger, you can access information related to the event by calling EventData() function. EventData() function would return a value of type XML. The base XML schema returned by the eventdata() function depends on the scope and event type.

iv) On Database: Applies the scope of a DDL trigger to the current database. If specified, the trigger fires whenever event_type or event_group happens in the current database.

v) All Server : Applies the scope of a DDL trigger to the current server. If specified, the trigger fires whenever event_type or event_group happens anywhere in the current server.

Lets create a sample trigger which would fire when ever an "Alter table" statement is executed in our database.

Script to Create a sample table

Create table test
(
sno int identity,
firstname varchar(25)
)
Go

Code snippet to create a sample DDL trigger

Create trigger trgAlterTableForDatabase
On database
For alter_table As
Select eventdata()
Print 'You are not authorized to execute Alter Table statement in this database.'
Rollback
Go

Try and alter the table structure

Alter table test alter column firstname varchar(50)

Result:

On executing the above "Alter table" statement we would be welcomed with the below error message.

(1 row(s) affected)
You are not authorized to execute Alter Table statement in this database.
Msg 3609, Level 16, State 2, Line 1
Transaction ended in trigger. Batch has been aborted.

As said earlier eventdata() would return a XML string and in our case it would like this:


ALTER_TABLE
2005-08-26T07:46:48.880
58
VadivelDB
VadivelDB\Administrator
dbo
AdventureWorks
dbo
test
TABLE


Alter table test alter column firstname varchar(50)



Clean up

drop trigger trgAlterTableForDatabase on database
drop table test

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