<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> &lt;= <replaceable>y</replaceable>
</synopsis>
    Notice that <token>BETWEEN</token> treats the endpoint values as included
    in the range.
    <literal>BETWEEN SYMMETRIC</literal> is like <literal>BETWEEN</literal>
    except there is no requirement that the argument to the left of
    <literal>AND</literal> be less than or equal to the argument on the right.
    If it is not, those two arguments are automatically swapped, so that
    a nonempty range is always implied.
   </para>

   <para>
    The various variants of <literal>BETWEEN</literal> are implemented in
    terms of the ordinary comparison operators, and therefore will work for
    any data type(s) that can be compared.
   </para>

   <note>
    <para>
     The use of <literal>AND</literal> in the <literal>BETWEEN</literal>
     syntax creates an ambiguity with the use of <literal>AND</literal> as a
     logical operator.  To resolve this, only a limited set of expression
     types are allowed as the second argument of a <literal>BETWEEN</literal>
     clause.  If you need to write a more complex sub-expression
     in <literal>BETWEEN</literal>, write parentheses around the
     sub-expression.
    </para>
   </note>

   <para>
    <indexterm>
     <primary>IS DISTINCT FROM</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT DISTINCT FROM</primary>
    </indexterm>
    Ordinary comparison operators yield null (signifying <quote>unknown</quote>),
    not true or false, when either input is null.  For example,
    <literal>7 = NULL</literal> yields null, as does <literal>7 &lt;&gt; NULL</literal>.  When
    this behavior is not suitable, use the
    <literal>IS <optional> NOT </optional> DISTINCT FROM</literal> predicates:
<synopsis>
<replaceable>a</replaceable> IS DISTINCT FROM <replaceable>b</replaceable>
<replaceable>a</replaceable> IS NOT DISTINCT FROM <replaceable>b</replaceable>
</synopsis>
    For non-null inputs, <literal>IS DISTINCT FROM</literal> is
    the same as the <literal>&lt;&gt;</literal> operator.  However, if both
    inputs are null it returns false, and if only one input is
    null it returns true.  Similarly, <literal>IS NOT DISTINCT
    FROM</literal> is identical to <literal>=</literal> for non-null
    inputs, but it returns true when both inputs are null, and false when only
    one input is null. Thus, these predicates effectively act as though null
    were a normal data value, rather than <quote>unknown</quote>.
   </para>

   <para>
    <indexterm>
     <primary>IS NULL</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT NULL</primary>
    </indexterm>
    <indexterm>
     <primary>ISNULL</primary>
    </indexterm>
    <indexterm>
     <primary>NOTNULL</primary>
    </indexterm>
    To check whether a value is or is not null, use the predicates:
<synopsis>
<replaceable>expression</replaceable> IS NULL
<replaceable>expression</replaceable> IS NOT NULL
</synopsis>
    or the equivalent, but nonstandard, predicates:
<synopsis>