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
INSTEAD OF UPDATE
Print (‘INSTEAD OF Trigger [trgInsteadOfTrigger1] – Trigger executed !!’)
CREATE TRIGGER trgInsteadOfTrigger2
INSTEAD OF UPDATE
Print (‘INSTEAD OF Trigger [trgInsteadOfTrigger2] – Trigger executed !!’)
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.
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.
Navneeth Diwaker Naik (email@example.com) is an employee of Satyam Computers Services Limited, Hyderabad, Andhra Pradesh. [INDIA].
Published with the explicit written permission of the author. Copyright 2004.