<!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/create_domain.sgml,v 1.34 2010/04/03 07:22:58 petere Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/create_domain.sgml,v 1.35 2010/05/29 19:06:16 momjian Exp $
PostgreSQL documentation
-->
<term><literal>NOT NULL</></term>
<listitem>
<para>
- Values of this domain are not allowed to be null.
+ Values of this domain are normally prevented from being null.
+ However, it is still possible for a domain with this constraint
+ to take a null value if it is assigned a matching domain type
+ that has become null, e.g. via a LEFT OUTER JOIN, or
+ <command>INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM
+ tab WHERE false))</command>.
</para>
</listitem>
</varlistentry>