<productname>PostgreSQL</productname> provides the
    standard <acronym>SQL</acronym> type <type>boolean</type>;
    see <xref linkend="datatype-boolean-table"/>.
    The <type>boolean</type> type can have several states:
    <quote>true</quote>, <quote>false</quote>, and a third state,
    <quote>unknown</quote>, which is represented by the
    <acronym>SQL</acronym> null value.
   </para>

   <table id="datatype-boolean-table">
    <title>Boolean Data Type</title>
    <tgroup cols="3">
     <thead>
      <row>
       <entry>Name</entry>
       <entry>Storage Size</entry>
       <entry>Description</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry><type>boolean</type></entry>
       <entry>1 byte</entry>
       <entry>state of true or false</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    Boolean constants can be represented in SQL queries by the SQL
    key words <literal>TRUE</literal>, <literal>FALSE</literal>,
    and <literal>NULL</literal>.
   </para>

   <para>
    The datatype input function for type <type>boolean</type> accepts these
    string representations for the <quote>true</quote> state:
    <simplelist>
     <member><literal>true</literal></member>
     <member><literal>yes</literal></member>
     <member><literal>on</literal></member>
     <member><literal>1</literal></member>
    </simplelist>
    and these representations for the <quote>false</quote> state:
    <simplelist>
     <member><literal>false</literal></member>
     <member><literal>no</literal></member>
     <member><literal>off</literal></member>
     <member><literal>0</literal></member>
    </simplelist>
    Unique prefixes of these strings are also accepted, for
    example <literal>t</literal> or <literal>n</literal>.
    Leading or trailing whitespace is ignored, and case does not matter.
   </para>

   <para>
    The datatype output function for type <type>boolean</type> always emits
    either <literal>t</literal> or <literal>f</literal>, as shown in
    <xref linkend="datatype-boolean-example"/>.
   </para>

   <example id="datatype-boolean-example">
    <title>Using the <type>boolean</type> Type</title>

<programlisting>
CREATE TABLE test1 (a boolean, b text);
INSERT INTO test1 VALUES (TRUE, 'sic est');
INSERT INTO test1 VALUES (FALSE, 'non est');
SELECT * FROM test1;
 a |    b
---+---------
 t | sic est
 f | non est

SELECT * FROM test1 WHERE a;
 a |    b
---+---------
 t | sic est
</programlisting>
   </example>

   <para>
    The key words <literal>TRUE</literal> and <literal>FALSE</literal> are
    the preferred (<acronym>SQL</acronym>-compliant) method for writing
    Boolean constants in SQL queries.  But you can also use the string
    representations by following the generic string-literal constant syntax
    described in <xref linkend="sql-syntax-constants-generic"/>, for
    example <literal>'yes'::boolean</literal>.
   </para>

   <para>
    Note that the parser automatically understands
    that <literal>TRUE</literal> and <literal>FALSE</literal> are of
    type <type>boolean</type>, but this is not so
    for <literal>NULL</literal> because that can have any type.
    So in some contexts you might have to cast <literal>NULL</literal>
    to <type>boolean</type> explicitly, for
    example <literal>NULL::boolean</literal>.  Conversely, the cast can be
    omitted from a string-literal Boolean value in contexts where the parser
    can deduce that the literal must be of type <type>boolean</type>.
   </para>
  </sect1>

  <sect1 id="datatype-enum">
   <title>Enumerated Types</title>

   <indexterm zone="datatype-enum">
    <primary>data type</primary>
    <secondary>enumerated (enum)</secondary>
   </indexterm>

   <indexterm zone="datatype-enum">
    <primary>enumerated types</primary>
   </indexterm>

   <para>
    Enumerated (enum) types are data types that
    comprise a static, ordered set of values.
    They are equivalent to the <type>enum</type>
    types supported in