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.
See Also:
Denial of Service; Distributed Denial of Service; Virus.
GNU Free Documentation License. Database Trigger. GNU Website.
http://en.wikipedia.org/wiki/Database_trigger.
