13.1. What Is a Trigger?
A trigger is a special kind of stored procedure that responds to specific events. There are two kinds of triggers: Data Definition Language (DDL) triggers and Data Manipulation Language (DML) triggers.
DDL triggers fire in response to someone changing the structure of your database in some way (CREATE, ALTER, DROP, and similar statements). These are new with SQL Server 2005 and are critical to some installations (particularly high-security installations) but are pretty narrow in use. In general, you will need to look into using these only where you need extreme auditing of changes/history of your database structure. We will save these until last.
DML triggers are pieces of code that you attach to a particular table or view. Unlike sprocs, where you needed to explicitly invoke the code, the code in triggers is automatically run whenever the event(s) you attached the trigger to occur in the table. Indeed, you can't explicitly invoke triggers — the only way to do this is by performing the required action in the table that they are assigned to.
Beyond not being able to explicitly invoke a trigger, you'll find two other things that exist for sprocs but are missing from triggers: parameters and return codes.
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 ...