value expression is one of the following:

   <itemizedlist>
    <listitem>
     <para>
      A constant or literal value
     </para>
    </listitem>

    <listitem>
     <para>
      A column reference
     </para>
    </listitem>

    <listitem>
     <para>
      A positional parameter reference, in the body of a function definition
      or prepared statement
     </para>
    </listitem>

    <listitem>
     <para>
      A subscripted expression
     </para>
    </listitem>

    <listitem>
     <para>
      A field selection expression
     </para>
    </listitem>

    <listitem>
     <para>
      An operator invocation
     </para>
    </listitem>

    <listitem>
     <para>
      A function call
     </para>
    </listitem>

    <listitem>
     <para>
      An aggregate expression
     </para>
    </listitem>

    <listitem>
     <para>
      A window function call
     </para>
    </listitem>

    <listitem>
     <para>
      A type cast
     </para>
    </listitem>

    <listitem>
     <para>
      A collation expression
     </para>
    </listitem>

    <listitem>
     <para>
      A scalar subquery
     </para>
    </listitem>

    <listitem>
     <para>
      An array constructor
     </para>
    </listitem>

    <listitem>
     <para>
      A row constructor
     </para>
    </listitem>

    <listitem>
     <para>
      Another value expression in parentheses (used to group
      subexpressions and override
      precedence<indexterm><primary>parenthesis</primary></indexterm>)
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   In addition to this list, there are a number of constructs that can
   be classified as an expression but do not follow any general syntax
   rules.  These generally have the semantics of a function or
   operator and are explained in the appropriate location in <xref
   linkend="functions"/>.  An example is the <literal>IS NULL</literal>
   clause.
  </para>

  <para>
   We have already discussed constants in <xref
   linkend="sql-syntax-constants"/>.  The following sections discuss
   the remaining options.
  </para>

  <sect2 id="sql-expressions-column-refs">
   <title>Column References</title>

   <indexterm>
    <primary>column reference</primary>
   </indexterm>

   <para>
    A column can be referenced in the form:
<synopsis>
<replaceable>correlation</replaceable>.<replaceable>columnname</replaceable>
</synopsis>
   </para>

   <para>
    <replaceable>correlation</replaceable> is the name of a
    table (possibly qualified with a schema name), or an alias for a table
    defined by means of a <literal>FROM</literal> clause.
    The correlation name and separating dot can be omitted if the column name
    is unique across all the tables being used in the current query.  (See also <xref linkend="queries"/>.)
   </para>
  </sect2>

  <sect2 id="sql-expressions-parameters-positional">
   <title>Positional Parameters</title>

   <indexterm>
    <primary>parameter</primary>
    <secondary>syntax</secondary>
   </indexterm>

   <indexterm>
    <primary>$</primary>
   </indexterm>

   <para>
    A positional parameter reference is used to indicate a value
    that is supplied externally to an SQL statement.  Parameters are
    used in SQL function definitions and in prepared queries.  Some
    client libraries also support specifying data values separately
    from the SQL command string, in which case parameters are used to
    refer to the out-of-line data values.
    The form of a parameter reference is:
<synopsis>
$<replaceable>number</replaceable>
</synopsis>
   </para>

   <para>
    For example, consider the definition of a function,
    <function>dept</function>, as:

<programlisting>
CREATE FUNCTION dept(text) RETURNS dept
    AS $$ SELECT * FROM dept WHERE name = $1 $$
    LANGUAGE SQL;
</programlisting>

    Here the <literal>$1</literal> references the value of the first
    function argument whenever the function is invoked.
   </para>
  </sect2>

  <sect2 id="sql-expressions-subscripts">