operation will fail.

</para>

<para>

The appropriate choice of <literal>ON DELETE</literal> action depends on

what kinds of objects the related tables represent. When the referencing

table represents something that is a component of what is represented by

the referenced table and cannot exist independently, then

<literal>CASCADE</literal> could be appropriate. If the two tables

represent independent objects, then <literal>RESTRICT</literal> or

<literal>NO ACTION</literal> is more appropriate; an application that

actually wants to delete both objects would then have to be explicit about

this and run two delete options. In the above example, order items are

part of an order, and it is convenient if they are deleted automatically

if an order is deleted. But products and orders are different things, and

so making a deletion of a product automatically cause the deletion of some

order items could be considered problematic. The actions <literal>SET

NULL</literal> or <literal>SET DEFAULT</literal> can be appropriate if a

foreign-key relationship represents optional information. For example, if

the products table contained a reference to a product manager, and the

product manager entry gets deleted, then setting the product's product

manager to null or a default might be useful.

</para>

<para>

Analogous to <literal>ON DELETE</literal> there is also

<literal>ON UPDATE</literal> which is invoked when a referenced