Trigger

A database trigger is procedural code that is 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 data modifications.

There are two classes of triggers, they are either "row triggers" or "statement triggers". With row triggers you can define an action for every row of a table, while statement triggers occur only once per INSERT, UPDATE, or DELETE statement. Triggers cannot be used to audit data retrieval via SELECT statements.

Each class can be of several types. There are "BEFORE triggers" and "AFTER triggers" which identifies the time of execution of the trigger. There is also an "INSTEAD OF trigger" which is a trigger that will execute instead of the triggering statement.

There are typically three triggering events that cause triggers to 'fire':

INSERT event (as a new record is being inserted into the database).
UPDATE event (as a record is being changed).
DELETE event (as a record is being deleted).
The trigger is used to automate DML condition process.

The major features and effects of database triggers are that they:

do not accept parameters or arguments (but may store affected-data in temporary tables)
cannot perform commit or rollback operations because they are part of the triggering SQL statement (only through autonomous transactions)
can cause mutating table errors, if they are poorly written.