A rule is a user-defined mechanism that invokes a database procedure whenever the database changes in a specified way, for example by insert, update, or delete.

A rule is a general-purpose mechanism that can be implemented for many purposes. For example, integrities can be implemented by rules. With integrities, violations are not specifically flagged or reported as errors. With rules, however, you can control exactly what happens when a violation occurs by defining in the database procedure the actions to take.