How to use a trigger in Oracle
When creating a trigger, consider how the trigger affects the data in the table. For example, if you wish to change the value of one column of data when inserting a row, your trigger should fire before the insert and should fire once per row. A trigger that is used to record who performs updates on a table can fire either before or after the update and should fire once per transaction.
A trigger is a pl/sql block structure which is fired when a DML statements like Insert, Delete, Update is executed on a database table. A trigger is triggered automatically when an associated DML statement is executed.
The Syntax for creating a trigger is:
CREATE [OR REPLACE ] TRIGGER trigger_name
{BEFORE | AFTER | INSTEAD OF }
{INSERT [OR] | UPDATE [OR] | DELETE}
[OF col_name]
ON table_name
[REFERENCING OLD AS o NEW AS n]
[FOR EACH ROW]
WHEN (condition)
BEGIN
--- sql statements
END;