An Introduction to Triggers -- Part II

By Garth Wells on 4 November 2001 | 1 Comment | Tags: Triggers

A few months ago I wrote an article for SQLTeam called An Introduction to Triggers -- Part I. The article covered trigger fundamentals--the most important being the way the inserted and deleted virtual tables work. I also showed an example of how to use an INSERT trigger to log activity on a web site. In this article I want to show how to use an UPDATE and DELETE trigger and introduce INSTEAD OF triggers. If you are not familiar with the inserted and deleted tables please read this article before continuing.

UPDATE Trigger

An UPDATE trigger is used to perform an action after an update is made on a table. The example shown here is based on the table used in Part I of this article.

CREATE TRIGGER tr_Orders_UPDATE
ON Orders
AFTER UPDATE
AS

--Make sure Priority was changed
IF NOT UPDATE(Ord_Priority)
 RETURN

--Determine if Priority was changed to high
IF EXISTS (SELECT *
           FROM inserted a
	   JOIN deleted b ON a.Ord_ID=b.Ord_ID
	   WHERE b.Ord_Priority <> 'High' AND
	   a.Ord_Priority = 'High')
 BEGIN
  DECLARE @Count tinyint
  SET @Count = (SELECT COUNT(*)
                FROM inserted a
		JOIN deleted b ON a.Ord_ID=b.Ord_ID
		WHERE b.Ord_Priority <> 'High' AND
		a.Ord_Priority = 'High')
  PRINT CAST(@Count as varchar(3))+' row(s) where changed to a priority of High'
 END
go

In Part I the INSERT trigger watched for orders with a priority of 'High.' The UPDATE trigger watches for orders whose priority are changed from something else to High.

The IF statement checks to see if the Ord_Priority value was changed. If not we can save some time by exiting the trigger immediately.

The deleted table holds the pre-UPDATE values and inserted table holds the new values. When the tables are joined it is easy to tell when the priority changes from something else to High.

DELETE Trigger

Once you understand how an UPDATE trigger works a DELETE trigger is easy to implement. In the example shown here it simply counts the number of rows in the deleted table to see how many had a priority of high.

CREATE TRIGGER tr_Orders_DELETE
ON Orders
AFTER DELETE
AS

--Determine if Order with a Priority of High was deleted
IF EXISTS (SELECT * FROM deleted WHERE Ord_Priority = 'High')
 BEGIN
  DECLARE @Count tinyint
  SET @Count = (SELECT * FROM deleted WHERE Ord_Priority = 'High')
  PRINT CAST(@Count as varchar(3))+' row(s) where deleted whose priority was High'
 END
go

INSTEAD OF Triggers

INSTEAD OF triggers are new to SQL Server 2000. The main reason they were introduced is to facilitate updating Views. I am going to show you how one works, but I am not going to bother with updating a View. Quite frankly I think updating Views is poor application design. I was once told by one of my editors that some of my views on Views reflect my "insular" experience, so if you do not agree with me on this point rest assured you are not alone.

An INSTEAD OF Trigger is used to perform an action instead of the one that caused the trigger to be fired. This sounds like double-talk, so I will explain a little more. Let's say you have an INSTEAD OF INSERT trigger defined on a table and an INSERT is executed. A row is not added to the table, but the code in the trigger is fired. The following shows what this looks like.

CREATE TRIGGER tr_Orders_INSERT_InsteadOf
ON Orders
INSTEAD OF INSERT
AS
PRINT 'Updateable Views are Messy'
go

The following INSERT produces the message shown, but the row is not added to the table.

INSERT Orders (Ord_Priority) VALUES ('High')

-- Results --

Updateable Views are Messy

Feel free to experiment with this type of trigger, but unless you are trying to update a View that is based on multiple tables I do not think it will be of much practical value.

Need More Information on Views?

If you would like more information on Views you can read a free chapter I have listed on my web site. Go to www.SQLBook.com and click on the Sample Chapters section. After reading the chapter you will really know how I feel about this pathetic excuse for a database object (just kidding).