</para>

    <para>
     The <literal>sql_standard</literal> style produces output that conforms to
     the SQL standard's specification for interval literal strings, if
     the interval value meets the standard's restrictions (either year-month
     only or day-time only, with no mixing of positive
     and negative components).  Otherwise the output looks like a standard
     year-month literal string followed by a day-time literal string,
     with explicit signs added to disambiguate mixed-sign intervals.
    </para>

    <para>
     The output of the <literal>postgres</literal> style matches the output of
     <productname>PostgreSQL</productname> releases prior to 8.4 when the
     <xref linkend="guc-datestyle"/> parameter was set to <literal>ISO</literal>.
    </para>

    <para>
     The output of the <literal>postgres_verbose</literal> style matches the output of
     <productname>PostgreSQL</productname> releases prior to 8.4 when the
     <varname>DateStyle</varname> parameter was set to non-<literal>ISO</literal> output.
    </para>

    <para>
     The output of the <literal>iso_8601</literal> style matches the <quote>format
     with designators</quote> described in section 4.4.3.2 of the
     ISO 8601 standard.
    </para>

     <table id="interval-style-output-table">
       <title>Interval Output Style Examples</title>
       <tgroup cols="4">
        <thead>
         <row>
          <entry>Style Specification</entry>
          <entry>Year-Month Interval</entry>
          <entry>Day-Time Interval</entry>
          <entry>Mixed Interval</entry>
         </row>
        </thead>
        <tbody>
         <row>
          <entry><literal>sql_standard</literal></entry>
          <entry>1-2</entry>
          <entry>3 4:05:06</entry>
          <entry>-1-2 +3 -4:05:06</entry>
         </row>
         <row>
          <entry><literal>postgres</literal></entry>
          <entry>1 year 2 mons</entry>
          <entry>3 days 04:05:06</entry>
          <entry>-1 year -2 mons +3 days -04:05:06</entry>
         </row>
         <row>
          <entry><literal>postgres_verbose</literal></entry>
          <entry>@ 1 year 2 mons</entry>
          <entry>@ 3 days 4 hours 5 mins 6 secs</entry>
          <entry>@ 1 year 2 mons -3 days 4 hours 5 mins 6 secs ago</entry>
         </row>
         <row>
          <entry><literal>iso_8601</literal></entry>
          <entry>P1Y2M</entry>
          <entry>P3DT4H5M6S</entry>
          <entry>P-1Y-2M3D&zwsp;T-4H-5M-6S</entry>
         </row>
        </tbody>
       </tgroup>
    </table>

   </sect2>

  </sect1>

  <sect1 id="datatype-boolean">
   <title>Boolean Type</title>

   <indexterm zone="datatype-boolean">
    <primary>Boolean</primary>
    <secondary>data type</secondary>
   </indexterm>

   <indexterm zone="datatype-boolean">
    <primary>true</primary>
   </indexterm>

   <indexterm zone="datatype-boolean">
    <primary>false</primary>
   </indexterm>

   <para>
    <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>