How Triggers Are Used

How Triggers Are Used
Triggers supplement the standard capabilities of Oracle to provide a highly customized database management system. For example, a trigger can restrict DML operations against a table to those issued during regular business hours. You can also use triggers to:
• Automatically generate derived column values
• Prevent invalid transactions
• Enforce complex security authorizations
• Enforce referential integrity across nodes in a distributed database
• Enforce complex business rules
• Provide transparent event logging
• Provide auditing
• Maintain synchronous table replicates
• Gather statistics on table access
• Modify table data when DML statements are issued against views
• Publish information about database events, user events, and SQL statements to subscribing applications

Types of Triggers
This section describes the different types of triggers:
• Row Triggers and Statement Triggers
• BEFORE and AFTER Triggers
• INSTEAD OF Triggers
• Triggers on System Events and User Events
Row Triggers and Statement Triggers
When you define a trigger, you can specify the number of times the trigger action is to be run:
• Once for every row affected by the triggering statement, such as a trigger fired by an UPDATE statement that updates many rows
• Once for the triggering statement, no matter how many rows it affects
Row Triggers
A row trigger is fired each time the table is affected by the triggering statement. For example, if an UPDATE statement updates multiple rows of a table, a row trigger is fired once for each row affected by the UPDATE statement. If a triggering statement affects no rows, a row trigger is not run.
Row triggers are useful if the code in the trigger action depends on data provided by the triggering statement or rows that are affected. For example, Figure 22-3 illustrates a row trigger that uses the values of each row affected by the triggering statement.
Statement Triggers
A statement trigger is fired once on behalf of the triggering statement, regardless of the number of rows in the table that the triggering statement affects, even if no rows are affected. For example, if a DELETE statement deletes several rows from a table, a statement-level DELETE trigger is fired only once.
Statement triggers are useful if the code in the trigger action does not depend on the data provided by the triggering statement or the rows affected. For example, use a statement trigger to:
• Make a complex security check on the current time or user
• Generate a single audit record
BEFORE and AFTER Triggers
When defining a trigger, you can specify the trigger timing—whether the trigger action is to be run before or after the triggering statement. BEFORE and AFTER apply to both statement and row triggers.
BEFORE and AFTER triggers fired by DML statements can be defined only on tables, not on views. However, triggers on the base tables of a view are fired if an INSERT, UPDATE, or DELETE statement is issued against the view. BEFORE and AFTER triggers fired by DDL statements can be defined only on the database or a schema, not on particular tables.