Procedural code automatically executed in response to certain events on a particular table in a database. Triggers can restrict access to specific data, perform logging, or audit access to data. Triggers can be either “row triggers” or “statement triggers.” The former define an action for every row of a table, whereas the latter occur only once and are not dependent on the shape of the data. Moreover, there are BEFORE and AFTER triggers, which alter the time of execution of the trigger. Also, an INSTEAD OF trigger, a conditional trigger, will fire instead of the triggering statement. There are typically three triggering events that cause trigger to fire: INSERT (as a new record is being inserted into the database); UPDATE (as a record is being changed); and DELETE (as a record is being deleted). Databases that support triggers typically give programmers access to record variables by means of a syntax such as OLD.cust_name or NEW.cust_name.
The term trigger is also used to describe an event that has to occur to activate a virus. These events can either be time based or condition based, meaning that one event must occur before the virus is activated. An example is the opening of a file or an email attachment. Other triggers can be activated over the network to coordinate a distributed attack, in which a number of hosts are infected with a virus and need to be activated at the same time to take down a target.
GNU Free Documentation License. Database Trigger. GNU Website. http://en.wikipedia.org/wiki/Database_trigger.