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2nd chunk of `doc/src/sgml/func.sgml`
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 <type>boolean</type> <returnvalue>boolean</returnvalue>
</synopsis>

    <acronym>SQL</acronym> uses a three-valued logic system with true,
    false, and <literal>null</literal>, which represents <quote>unknown</quote>.
    Observe the following truth tables:

    <informaltable>
     <tgroup cols="4">
      <thead>
       <row>
        <entry><replaceable>a</replaceable></entry>
        <entry><replaceable>b</replaceable></entry>
        <entry><replaceable>a</replaceable> AND <replaceable>b</replaceable></entry>
        <entry><replaceable>a</replaceable> OR <replaceable>b</replaceable></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>TRUE</entry>
        <entry>TRUE</entry>
        <entry>TRUE</entry>
        <entry>TRUE</entry>
       </row>

       <row>
        <entry>TRUE</entry>
        <entry>FALSE</entry>
        <entry>FALSE</entry>
        <entry>TRUE</entry>
       </row>

       <row>
        <entry>TRUE</entry>
        <entry>NULL</entry>
        <entry>NULL</entry>
        <entry>TRUE</entry>
       </row>

       <row>
        <entry>FALSE</entry>
        <entry>FALSE</entry>
        <entry>FALSE</entry>
        <entry>FALSE</entry>
       </row>

       <row>
        <entry>FALSE</entry>
        <entry>NULL</entry>
        <entry>FALSE</entry>
        <entry>NULL</entry>
       </row>

       <row>
        <entry>NULL</entry>
        <entry>NULL</entry>
        <entry>NULL</entry>
        <entry>NULL</entry>
       </row>
      </tbody>
     </tgroup>
    </informaltable>

    <informaltable>
     <tgroup cols="2">
      <thead>
       <row>
        <entry><replaceable>a</replaceable></entry>
        <entry>NOT <replaceable>a</replaceable></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry>TRUE</entry>
        <entry>FALSE</entry>
       </row>

       <row>
        <entry>FALSE</entry>
        <entry>TRUE</entry>
       </row>

       <row>
        <entry>NULL</entry>
        <entry>NULL</entry>
       </row>
      </tbody>
     </tgroup>
    </informaltable>
   </para>

   <para>
    The operators <literal>AND</literal> and <literal>OR</literal> are
    commutative, that is, you can switch the left and right operands
    without affecting the result.  (However, it is not guaranteed that
    the left operand is evaluated before the right operand.  See <xref
    linkend="syntax-express-eval"/> for more information about the
    order of evaluation of subexpressions.)
   </para>
  </sect1>

  <sect1 id="functions-comparison">
   <title>Comparison Functions and Operators</title>

   <indexterm zone="functions-comparison">
    <primary>comparison</primary>
    <secondary>operators</secondary>
   </indexterm>

   <para>
    The usual comparison operators are available, as shown in <xref
    linkend="functions-comparison-op-table"/>.
   </para>

   <table id="functions-comparison-op-table">
    <title>Comparison Operators</title>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>Operator</entry>
       <entry>Description</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>&lt;</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Less than</entry>
      </row>

      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>&gt;</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Greater than</entry>
      </row>

      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>&lt;=</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
       </entry>
       <entry>Less than or equal to</entry>
      </row>

      <row>
       <entry>
        <replaceable>datatype</replaceable> <literal>&gt;=</literal> <replaceable>datatype</replaceable>
        <returnvalue>boolean</returnvalue>
Title: Logical and Comparison Operators in PostgreSQL
Summary
This section provides truth tables for the logical operators AND, OR, and NOT in PostgreSQL, considering the three-valued logic (TRUE, FALSE, NULL). It notes that AND and OR are commutative. Following this, it introduces the standard comparison operators (<, >, <=, >=, =, !=) available in PostgreSQL, presenting them in a table format.