Test whether value is not null (nonstandard syntax).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS TRUE</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields true.
       </para>
       <para>
        <literal>true IS TRUE</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS TRUE</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS NOT TRUE</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields false or unknown.
       </para>
       <para>
        <literal>true IS NOT TRUE</literal>
        <returnvalue>f</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS NOT TRUE</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS FALSE</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields false.
       </para>
       <para>
        <literal>true IS FALSE</literal>
        <returnvalue>f</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS FALSE</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS NOT FALSE</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields true or unknown.
       </para>
       <para>
        <literal>true IS NOT FALSE</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS NOT FALSE</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS UNKNOWN</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields unknown.
       </para>
       <para>
        <literal>true IS UNKNOWN</literal>
        <returnvalue>f</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS UNKNOWN</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>boolean</type> <literal>IS NOT UNKNOWN</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether boolean expression yields true or false.
       </para>
       <para>
        <literal>true IS NOT UNKNOWN</literal>
        <returnvalue>t</returnvalue>
       </para>
       <para>
        <literal>NULL::boolean IS NOT UNKNOWN</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    <indexterm>
     <primary>BETWEEN</primary>
    </indexterm>
    <indexterm>
     <primary>BETWEEN SYMMETRIC</primary>
    </indexterm>
    The <token>BETWEEN</token> predicate simplifies range tests:
<synopsis>
<replaceable>a</replaceable> BETWEEN <replaceable>x</replaceable> AND <replaceable>y</replaceable>
</synopsis>
    is equivalent to
<synopsis>
<replaceable>a</replaceable> &gt;= <replaceable>x</replaceable> AND <replaceable>a</replaceable>