Reasons to Use Triggers

Triggers let you customize your database management system.

For example, you can use triggers to:

Automatically generate virtual column values
Log events
Gather statistics on table access
Modify table data when DML statements are issued against views
Enforce referential integrity when child and parent tables are on different nodes of a distributed database
Publish information about database events, user events, and SQL statements to subscribing applications
Prevent DML operations on a table after regular business hours
Prevent invalid transactions
Enforce complex business or referential integrity rules that you cannot define with constraints (see "How Triggers and Constraints Differ")

Caution:

Triggers are not reliable security mechanisms, because they are programmatic and easy to disable. For high-assurance security, use Oracle Database Vault, described in Oracle Database Vault Administrator's Guide.

How Triggers and Constraints Differ

Both triggers and constraints can constrain data input, but they differ significantly.

A trigger always applies to new data only. For example, a trigger can prevent a DML statement from inserting a NULL value into a database column, but the column might contain NULL values that were inserted into the column before the trigger was defined or while the trigger was disabled.

A constraint can apply either to new data only (like a trigger) or to both new and existing data. Constraint behavior depends on constraint state, as explained in Oracle Database SQL Language Reference.

Constraints are easier to write and less error-prone than triggers that enforce the same rules. However, triggers can enforce some complex business rules that constraints cannot. Oracle strongly recommends that you use triggers to constrain data input only in these situations:

To enforce referential integrity when child and parent tables are on different nodes of a distributed database
To enforce complex business or referential integrity rules that you cannot define with constraints