Oracle PL/SQL for DBAs

Database Triggers

Database triggers are named program units that are executed in response to events that occur in the database. Five different types of events can have trigger code attached to them:

Data Manipulation Language (DML) statements

DML triggers are available to fire whenever a record is inserted into, updated in, or deleted from a table. These triggers can be used to perform validation, set default values, audit changes, and even disallow certain DML operations.

Data Definition Language (DDL) statements

DDL triggers fire whenever DDL is executed—for example, whenever a table is created. These triggers can perform auditing and prevent certain DDL statements from occurring.

Database events

Database event triggers fire whenever the database starts up or is shut down, whenever a user logs on or off, and whenever an Oracle error occurs. For Oracle8i Database and above, these triggers provide a means of tracking activity in the database.

INSTEAD OF

INSTEAD OF triggers are essentially alternatives to DML triggers . They fire when inserts, updates, and deletes are about to occur; your code specifies what to do in place of these DML operations. INSTEAD OF triggers control operations on views, not tables. They can be used to make non-updateable views updateable and to override the behavior of views that are updateable. These triggers are not covered further in this section, as they are a specialized topic that requires thorough coverage to be useful.