PostgreSQL Question:

Answer #1
By SQL query you can trigger an event. Triggers can be activated with the help of INSERT and UPDATE queries. These can be attached to tables. Triggers more than one can be triggered alphabetically. These triggers have the capability to invoke functions from other languages.

Answer #2
Triggers are functions which are fired at two kinds of events,viz Row level events and Statement level. Further they can executed befor event or after event. A trigger returning Null will rollback the event changes.