Databasetriggers are named program units that are executed in response to events that occur in the database. Triggers are critical elements of a well-designed application built on the Oracle database, and are used to do the following:
Because the validation logic is attached directly to the database object, database triggers offer a strong guarantee that the required logic will always be executed and enforced.
Starting with Oracle8i, you can use database startup and shutdown triggers to automatically perform necessary initialization and cleanup steps. This is a distinct advantage over creating and running such steps as scripts external to the database.
You can use triggers to tightly control what kinds of actions are allowed on database objects, such as dropping or altering tables. Again, by putting this logic in triggers, you make it very difficult, if not impossible, for anyone to bypass the rules you have established.
Five different types of events can have trigger code attached to them:
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.