Beginning Microsoft SQL Server 2008 ... - S3 Tech Training

Another upshot of this part-of-the-same-transaction business is that triggers inherit the locks already
open on the transaction they are part of. This means that we don’t have to do anything special to make
sure that we don’t bump into the locks created by the other statements in the transaction. We have free
access within the scope of the transaction, and we see the database based on the modifications already
placed by previous statements within the transaction.
Using IF UPDATE() and COLUMNS_UPDATED
In an UPDATE trigger, we can often limit the amount of code that actually executes within the trigger by
checking to see whether the column(s) we are interested in are the ones that have been changed. To do
this, we make use of the UPDATE() or COLUMNS_UPDATED() functions. Let’s look at each.
The UPDATE() Function
The UPDATE() function has relevance only within the scope of a trigger. Its sole purpose in life is to provide
a boolean response (true/false) to whether a particular column has been updated or not. You can
use this function to decide whether a particular block of code needs to run or not — for example, if that
code is relevant only when a particular column is updated.
Let’s run a quick example of this by modifying one of our earlier triggers.
ALTER TRIGGER Production.ProductIsRationed
ON Production.ProductInventory
FOR UPDATE
AS
IF UPDATE(Quantity)
BEGIN
IF EXISTS
(
SELECT ‘True’
FROM Inserted i
JOIN Deleted d
ON i.ProductID = d.ProductID
AND i.LocationID = d.LocationID
WHERE (d.Quantity - i.Quantity) > d.Quantity / 2
AND d.Quantity > 0
)
BEGIN
RAISERROR(‘Cannot reduce stock by more than 50%% at once.’,16,1)
ROLLBACK TRAN
END
END
Chapter 15: Triggers
With this change, we will now limit the rest of the code to run only when the Quantity column (the one
we care about) has been changed. The user can change the value of any other column and we don’t care.
This means that we’ll be executing fewer lines of code and, therefore, this trigger will perform slightly
better than our previous version.
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