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

Chapter 15: Triggers
FOR|AFTER
The FOR (or, alternatively, you can use AFTER) clause indicates under what type of action(s) you want
this trigger to fire. You can have the trigger fire whenever there is an INSERT, UPDATE, or DELETE, or any
mix of the three. So, for example, your FOR clause could look something like:
… or:
… or:
FOR INSERT, DELETE
FOR UPDATE, INSERT
FOR DELETE
As stated in the section about the ON clause, triggers declared using the FOR or AFTER clause can be
attached only to tables — no views are allowed (see INSTEAD OF triggers for those).
INSERT Trigger
The code for any trigger that you mark as being FOR INSERT will be executed anytime that someone
inserts a new row into your table. For each row that is inserted, SQL Server will create a copy of that new
row and insert it in a special table that exists only within the scope of your trigger. That table is called
Inserted, and we’ll see much more of it over the course of this chapter. The big thing to understand is
that the Inserted table lives only as long as your trigger does. Think of it as not existing before your
trigger starts or after your trigger completes.
DELETE Trigger
This works much the same as an INSERT trigger does, save that the Inserted table will be empty (after
all, you deleted rather than inserted, so there are no records for the Inserted table). Instead, a copy of
each record that was deleted is inserted into another table called Deleted. That table, like the Inserted
table, is limited in scope to just the life of your trigger.
UPDATE Trigger
More of the same, save for a twist. The code in a trigger declared as being FOR UPDATE will be fired
whenever an existing record in your table is changed. The twist is that there’s no such table as UPDATED.
Instead, SQL Server treats each row as if the existing record had been deleted and a totally new record
was inserted. As you can probably guess from that, a trigger declared as FOR UPDATE contains not one
but two special tables called Inserted and Deleted. The two tables have exactly the same number of
rows, of course.
WITH APPEND
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WITH APPEND is something of an oddball and, in all honesty, you’re pretty unlikely to use it; nonetheless,
we’ll touch on it briefly. WITH APPEND applies only when you are running in 6.5 compatibility
mode (which can be set using sp_dbcmptlevel) — something that should be exceedingly rare at this
point (it’s been out of date for nearly a decade now).