Specify BEFORE to cause the database to fire the trigger before executing the triggering event. For row triggers, the trigger is fired before each affected row is changed.Restrictions on BEFORE Triggers
You cannot specify a BEFORE trigger on a view or an object view.
You can write to the :NEW value but not to the :OLD value.
Specify AFTER to cause the database to fire the trigger after executing the triggering event. For row triggers, the trigger is fired after each affected row is changed.Restrictions on AFTER Triggers
You cannot specify an AFTER trigger on a view or an object view.
You cannot write either the :OLD or the :NEW value.
Note:When you create a materialized view log for a table, Oracle Database implicitly creates an AFTER ROW trigger on the table. This trigger inserts a row into the materialized view log whenever an INSERT, UPDATE, or DELETE statement modifies data in the master table. You cannot control the order in which multiple row triggers fire. Therefore, you should not write triggers intended to affect the content of the materialized view.
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