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5th chunk of `doc/src/sgml/func.sgml`
a9030ab4d58ce618806c564cc35d68492f051ef4fe6ac9460000000100000fa0
 <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Not between, after sorting the two endpoint values.
       </para>
       <para>
        <literal>2 NOT BETWEEN SYMMETRIC 3 AND 1</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>IS DISTINCT FROM</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Not equal, treating null as a comparable value.
       </para>
       <para>
        <literal>1 IS DISTINCT FROM NULL</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para>
       <para>
        <literal>NULL IS DISTINCT FROM NULL</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>IS NOT DISTINCT FROM</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Equal, treating null as a comparable value.
       </para>
       <para>
        <literal>1 IS NOT DISTINCT FROM NULL</literal>
        <returnvalue>f</returnvalue> (rather than <literal>NULL</literal>)
       </para>
       <para>
        <literal>NULL IS NOT DISTINCT FROM NULL</literal>
        <returnvalue>t</returnvalue> (rather than <literal>NULL</literal>)
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>IS NULL</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether value is null.
       </para>
       <para>
        <literal>1.5 IS NULL</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>IS NOT NULL</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether value is not null.
       </para>
       <para>
        <literal>'null' IS NOT NULL</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>ISNULL</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        Test whether value is null (nonstandard syntax).
       </para></entry>
      </row>

      <row>
       <entry role="func_table_entry"><para role="func_signature">
        <replaceable>datatype</replaceable> <literal>NOTNULL</literal>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        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>
Title: Comparison Predicates: IS (NOT) DISTINCT FROM, IS (NOT) NULL, IS (NOT) TRUE
Summary
This section describes further comparison predicates in PostgreSQL. It covers IS NOT DISTINCT FROM (equal, treating null as comparable), IS NULL (tests for null value), IS NOT NULL (tests for non-null value), IS TRUE (tests if a boolean expression yields true), and IS NOT TRUE (tests if a boolean expression yields false or unknown). It also mentions the non-standard syntax predicates ISNULL and NOTNULL.