BEFORE and AFTER Triggers

BEFORE and AFTER Triggers
When defining a trigger, you can specify the trigger timing—whether the trigger action is to be run before or after the triggering statement. BEFORE and AFTER apply to both statement and row triggers.
BEFORE and AFTER triggers fired by DML statements can be defined only on tables, not on views. However, triggers on the base tables of a view are fired if an INSERT, UPDATE, or DELETE statement is issued against the view. BEFORE and AFTER triggers fired by DDL statements can be defined only on the database or a schema, not on particular tables.
BEFORE Triggers
BEFORE triggers run the trigger action before the triggering statement is run. This type of trigger is commonly used in the following situations:
• When the trigger action determines whether the triggering statement should be allowed to complete. Using a BEFORE trigger for this purpose, you can eliminate unnecessary processing of the triggering statement and its eventual rollback in cases where an exception is raised in the trigger action.
• To derive specific column values before completing a triggering INSERT or UPDATE statement.
AFTER Triggers
AFTER triggers run the trigger action after the triggering statement is run.
Trigger Type Combinations
Using the options listed previously, you can create four types of row and statement triggers:
• BEFORE statement trigger
Before executing the triggering statement, the trigger action is run.
• BEFORE row trigger
Before modifying each row affected by the triggering statement and before checking appropriate integrity constraints, the trigger action is run, if the trigger restriction was not violated.
• AFTER statement trigger
After executing the triggering statement and applying any deferred integrity constraints, the trigger action is run.
• AFTER row trigger
After modifying each row affected by the triggering statement and possibly applying appropriate integrity constraints, the trigger action is run for the current row provided the trigger restriction was not violated. Unlike BEFORE row triggers, AFTER row triggers lock rows.