<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>
<replaceable>expression</replaceable> ISNULL
<replaceable>expression</replaceable> NOTNULL
</synopsis>
    <indexterm><primary>null value</primary><secondary>comparing</secondary></indexterm>
   </para>

   <para>
    Do <emphasis>not</emphasis> write
    <literal><replaceable>expression</replaceable> = NULL</literal>
    because <literal>NULL</literal> is not <quote>equal to</quote>
    <literal>NULL</literal>.  (The null value represents an unknown value,
    and it is not known whether two unknown values are equal.)
   </para>

  <tip>
   <para>
    Some applications might expect that
    <literal><replaceable>expression</replaceable> = NULL</literal>
    returns true if <replaceable>expression</replaceable> evaluates to
    the null value.  It is highly recommended that these applications
    be modified to comply with the SQL standard. However, if that
    cannot be done the <xref linkend="guc-transform-null-equals"/>
    configuration variable is available. If it is enabled,
    <productname>PostgreSQL</productname> will convert <literal>x =
    NULL</literal> clauses to <literal>x IS NULL</literal>.
   </para>
  </tip>

   <para>
    If the <replaceable>expression</replaceable> is row-valued, then
    <literal>IS NULL</literal> is true when the row expression itself is null
    or when all the row's fields are null, while
    <literal>IS NOT NULL</literal> is true when the row expression itself is non-null
    and all the row's fields are non-null.  Because of this behavior,
    <literal>IS NULL</literal> and <literal>IS NOT NULL</literal> do not always return
    inverse results for row-valued expressions; in particular, a row-valued
    expression that contains both null and non-null fields will return false
    for both tests.  For example:

<programlisting>
SELECT ROW(1,2.5,'this is a test') = ROW(1, 3, 'not the same');

SELECT ROW(table.*) IS NULL FROM table;  -- detect all-null rows

SELECT ROW(table.*) IS NOT NULL FROM table;  -- detect all-non-null rows

SELECT NOT(ROW(table.*) IS NOT NULL) FROM TABLE; -- detect at least one null in rows
</programlisting>

    In some cases, it may be preferable to
    write <replaceable>row</replaceable> <literal>IS DISTINCT FROM NULL</literal>
    or <replaceable>row</replaceable> <literal>IS NOT DISTINCT FROM NULL</literal>,
    which will simply check whether the overall row value is null without any
    additional tests on the row fields.
   </para>

   <para>
    <indexterm>
     <primary>IS TRUE</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT TRUE</primary>
    </indexterm>
    <indexterm>
     <primary>IS FALSE</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT FALSE</primary>
    </indexterm>
    <indexterm>
     <primary>IS UNKNOWN</primary>
    </indexterm>
    <indexterm>
     <primary>IS NOT UNKNOWN</primary>
    </indexterm>
    Boolean values can also be tested using the predicates
<synopsis>
<replaceable>boolean_expression</replaceable> IS TRUE
<replaceable>boolean_expression</replaceable>