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Postgres documentation

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two rules have the same semantics:

<programlisting>

ON UPDATE TO emp.salary WHERE emp.name = "Joe"

DO UPDATE emp ( ... ) WHERE ...

DO

UPDATE emp ( ... ) WHERE ...

</programlisting>

<programlisting>

ON UPDATE TO emp-1.salary WHERE emp-2.name = "Joe"

DO UPDATE emp-3 ( ... ) WHERE ...

DO

UPDATE emp-3 ( ... ) WHERE ...

</programlisting>

Each rule can have the optional tag INSTEAD.

<programlisting>

CREATE RULE bad_rule_combination_1 AS

ON SELECT TO emp

DO INSTEAD SELECT TO toyemp;

DO INSTEAD

SELECT TO toyemp;

</programlisting>

<programlisting>

CREATE RULE bad_rule_combination_2 AS

ON SELECT TO toyemp

DO INSTEAD SELECT TO emp;

DO INSTEAD

SELECT TO emp;

</programlisting>

<para>

This attempt to retrieve from EMP will cause

<programlisting>

CREATE RULE example_1 AS

ON UPDATE emp.salary WHERE current.name = "Joe"

DO UPDATE emp (salary = new.salary)

WHERE emp.name = "Sam";

DO

UPDATE emp

SET salary = new.salary

WHERE emp.name = "Sam";

</programlisting>

At the time Joe receives a salary adjustment, the event

ON SELECT TO EMP.salary

WHERE current.name = "Bill"

DO INSTEAD

SELECT (emp.salary) from emp

WHERE emp.name = "Joe";

SELECT emp.salary

FROM emp

WHERE emp.name = "Joe";

</programlisting>

</para>

<para>

the current user):

<programlisting>

CREATE RULE example_3 AS

ON SELECT TO emp.salary

WHERE current.dept = "shoe" AND current_user = "Joe"

ON

SELECT TO emp.salary

WHERE current.dept = "shoe" AND current_user = "Joe"

DO INSTEAD NOTHING;

</programlisting>

</para>

CREATE RULE example_4 AS

ON SELECT TO toyemp

DO INSTEAD

SELECT (emp.name, emp.salary) FROM emp

SELECT (emp.name, emp.salary)

FROM emp

WHERE emp.dept = "toy";

</programlisting>

</para>

<programlisting>

CREATE RULE example_5 AS

ON INERT TO emp WHERE new.salary > 5000

DO UPDATE NEWSET salary = 5000;

DO

UPDATE NEWSET salary = 5000;

</programlisting>

</para>

</refsect1>