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

Chapter 15: Triggers
460
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 – i.Quantity > 0
)
BEGIN
RAISERROR(‘Cannot reduce stock by more than 50%% at once.’,16,1)
ROLLBACK TRAN
END
Before we test this, let’s analyze what we’re doing here.
First, we’re making use of an IF EXISTS just as we have throughout this chapter. We want to do the
rollback only if something exists that meets the evil, mean, and nasty criteria that we’ll be testing for.
Then we join the Inserted and Deleted tables together — this is what gives us the chance to compare
the two.
Our WHERE clause is the point where things might become a bit confusing. The first line of it is pretty
straightforward. It implements the nominal statement of our business requirement; updates to the
Quantity column that are more than half the units we previously had on hand will meet the EXISTS
criterion and cause the conditional code to run (rejecting the transaction).
The next line, though, is not quite so straightforward. As with all things in programming, we need to
think beyond the nominal statement of the problem, and think about other ramifications. The requirement
really applies only to reductions in orders — we certainly don’t want to restrict how many units
are put in stock — so we make sure that we worry only about updates where the number in stock after
the update is less than before the update. The > 0 requirement also addresses the scenario where there
was only one item in stock. In such a scenario, we want to go ahead and sell that last one, but 1-1 is greater
than ½, and the trigger would have rejected it without our > 0 addition.
If both of these conditions have been met (over 50 percent and a reduction, rather than addition, to the
inventory), then we raise the error. Notice the use of two % signs, rather than one, in RAISERROR. Remember
that a % works as a placeholder for a parameter, so one % by itself won’t show up when your error message
comes out. By putting two in a row (%%), we let SQL Server know that we really did want to print
out a percent sign.
OK — let’s check how it works. We’ll just pick a record and try to do an update that reduces the stock by
more than 50 percent:
UPDATE Production.ProductInventory
SET Quantity = 1 -- Was 408 if you want to set it back
WHERE ProductID = 1
AND LocationID = 1