referenced table is used as the referenced column(s).

</para>

<para>

You can assign your own name for a foreign key constraint,

in the usual way.

</para>

<para>

A foreign key can also constrain and reference a group of columns.

As usual, it then needs to be written in table constraint form.

match the number and type of the referenced columns.

</para>

<indexterm>

<primary>foreign key</primary>

<secondary>self-referential</secondary>

</indexterm>

<para>

You can assign your own name for a foreign key constraint,

in the usual way.

Sometimes it is useful for the <quote>other table</quote> of a

foreign key constraint to be the same table; this is called

a <firstterm>self-referential</firstterm> foreign key. For

example, if you want rows of a table to represent nodes of a tree

structure, you could write

<programlisting>

CREATE TABLE tree (

node_id integer PRIMARY KEY,

parent_id integer REFERENCES tree,

name text,

...

);

</programlisting>

A top-level node would have NULL <structfield>parent_id</structfield>,

but non-NULL <structfield>parent_id</structfield> entries would be

constrained to reference valid rows of the table.

</para>

<para>