EnterpriseDB: Rules Oracle Tips by Burleson Consulting

A rule can be thought of as a special type of trigger.  In most instances where it is possible, I would recommend you use a trigger instead of a rule.  The benefit of a rule instead of a trigger is that the rule can be an INSTEAD of rule which triggers do not support.

A rule can be an ALSO rule or an INSTEAD rule.  An ALSO rule will fire a set of commands in addition to the executing command while an INSTEAD rule will fire instead of the executing command.

A restriction on a select rule is that it can only be an INSTEAD rule and must consist of a single select.  It is a better practice to create a view instead of a select rule.

There are six valid commands: NOTHING, NOTIFY, INSERT, UPDATE, DELETE, SELECT.  INSERT, UPDATE, DELETE and SELECT perform as expected.  NOTHING does just that, absolutely nothing.  If you have an INSERT INSTEAD trigger that does NOTHING, you will never be able to add records to the table.  NOTIFY is a database command that pushes a message on a queue. It works a lot like DBMS_ALERT in Oracle.  I will discuss NOTIFY in detail in the next chapter.

CREATE RULE

CREATE OR REPLACE RULE <rule name>
            AS ON SELECT|INSERT|UPDATE|DELETE
            TO <table name>
            [ WHERE <where clause> ]
            DO [ ALSO | INSTEAD ]
            { NOTHING | command(s) }

We can create a rule on the emp table that will also insert data into the emp clone table whenever a record is added to emp.

CREATE OR REPLACE emp_insert_audit_rule
  AS ON INSERT TO emp
DO ALSO INSERT INTO emp_clone (empno, ename)
  VALUES (:new.empno, :new.ename);

ALTER RULE

You cannot alter a rule.  You should recreate it.

GRANT RULE

You cannot grant to a rule.  You would grant to the table that has the rule.

REVOKE RULE

You cannot revoke permissions from a rule.  You would revoke permissions from a table that has the rule.

DROP RULE

Drop a rule:

DROP RULE emp_insert_audit_rule;

This is an excerpt from the book "EnterpriseDB: The Definitive Reference" by Rampant TechPress.