Triggers
Enable the database programmer to specify:
- when to check a constraint,
- An event (e.g., update to an attribute)
- A condition (e.g., a query to check)
- An action (deletion, update, insertion)
When the event happens, the system will check the constraint, and
if satisfied, will perform the action.
NOTE: triggers may cause cascading effects.
Database vendors did not wait for standards with triggers!