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ON
While triggers take no parameters, they do have a mechanism for figuring out what records they are
supposed to act on (we’ll investigate this further later in the chapter). And, while you can use the
RETURN keyword, you cannot return a specific return code (because you didn’t explicitly call the trigger,
what would you return a return code to?).
What events can you attach triggers to? The three action query types you use in SQL. So, there are three
types of triggers, plus hybrids that come from mixing and matching the events and timing that fire them:
❑ INSERT triggers
❑ DELETE triggers
❑ UPDATE triggers
❑ A mix and match of any of the above
It’s worth noting that there are times when a trigger will not fire, even though it seems that the action
you are performing falls into one of the preceding categories. At issue is whether the operation you are
doing is in a logged activity or not. For example, a DELETE statement is a normal, logged activity that
would fire any delete trigger, but a TRUNCATE TABLE, which has the effect of deleting rows, just deallocates
the space used by the table — there is no individual deletion of rows logged, and no trigger is
fired. Bulk operations will not, by default, fire triggers (you can explicitly tell the bulk operation to fire
the triggers).
The syntax for creating triggers looks an awful lot like all of our other CREATE syntax examples, except,
as with a table, it has to be attached to a table — a trigger can’t stand on its own.
Let’s take a look:
CREATE TRIGGER
ON [.]
[WITH ENCRYPTION | EXECUTE AS ]
{{{FOR|AFTER} } |INSTEAD OF}
[WITH APPEND]
[NOT FOR REPLICATION]
AS
< | EXTERNAL NAME >
Chapter 15: Triggers
As you can see, the all too familiar CREATE is still there, as well as
the execution stuff we’ve seen in many other objects — we’ve just added the ON clause to indicate the
table to which this trigger is going to be attached, as well as when and under what conditions it fires.
This part just names the object you are creating the trigger against. Keep in mind that, if the type of the
trigger is an AFTER trigger (if it uses FOR or AFTER to declare the trigger), then the target of the ON clause
must be a table — AFTER triggers are not supported for views.
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