<literal>FROM</literal> clause, or the aliases given to them as
     explained in <xref linkend="queries-table-aliases"/>.  The name
     space available in the select list is the same as in the
     <literal>WHERE</literal> clause, unless grouping is used, in which case
     it is the same as in the <literal>HAVING</literal> clause.
   </para>

   <para>
    If more than one table has a column of the same name, the table
    name must also be given, as in:
<programlisting>
SELECT tbl1.a, tbl2.a, tbl1.b FROM ...
</programlisting>
    When working with multiple tables, it can also be useful to ask for
    all the columns of a particular table:
<programlisting>
SELECT tbl1.*, tbl2.a FROM ...
</programlisting>
    See <xref linkend="rowtypes-usage"/> for more about
    the <replaceable>table_name</replaceable><literal>.*</literal> notation.
   </para>

   <para>
    If an arbitrary value expression is used in the select list, it
    conceptually adds a new virtual column to the returned table.  The
    value expression is evaluated once for each result row, with
    the row's values substituted for any column references.  But the
    expressions in the select list do not have to reference any
    columns in the table expression of the <literal>FROM</literal> clause;
    they can be constant arithmetic expressions, for instance.
   </para>
  </sect2>

  <sect2 id="queries-column-labels">
   <title>Column Labels</title>

   <indexterm zone="queries-column-labels">
    <primary>alias</primary>
    <secondary>in the select list</secondary>
   </indexterm>

   <para>
    The entries in the select list can be assigned names for subsequent
    processing, such as for use in an <literal>ORDER BY</literal> clause
    or for display by the client application.  For example:
<programlisting>
SELECT a AS value, b + c AS sum FROM ...
</programlisting>
   </para>

   <para>
    If no output column name is specified using <literal>AS</literal>,
    the system assigns a default column name.  For simple column references,
    this is the name of the referenced column.  For function
    calls, this is the name of the function.  For complex expressions,
    the system will generate a generic name.
   </para>

   <para>
    The <literal>AS</literal> key word is usually optional, but in some
    cases where the desired column name matches a
    <productname>PostgreSQL</productname> key word, you must write
    <literal>AS</literal> or double-quote the column name in order to
    avoid ambiguity.
    (<xref linkend="sql-keywords-appendix"/> shows which key words
    require <literal>AS</literal> to be used as a column label.)
    For example, <literal>FROM</literal> is one such key word, so this
    does not work:
<programlisting>
SELECT a from, b + c AS sum FROM ...
</programlisting>
    but either of these do:
<programlisting>
SELECT a AS from, b + c AS sum FROM ...
SELECT a "from", b + c AS sum FROM ...
</programlisting>
    For greatest safety against possible
    future key word additions, it is recommended that you always either
    write <literal>AS</literal> or double-quote the output column name.
   </para>

   <note>
    <para>
     The naming of output columns here is different from that done in
     the <literal>FROM</literal> clause (see <xref
     linkend="queries-table-aliases"/>).  It is possible
     to rename the same column twice, but the name assigned in
     the select list is the one that will be passed on.
    </para>
   </note>
  </sect2>

  <sect2 id="queries-distinct">
   <title><literal>DISTINCT</literal></title>

   <indexterm zone="queries-distinct">
    <primary>ALL</primary>
    <secondary>SELECT ALL</secondary>
   </indexterm>
   <indexterm zone="queries-distinct">
    <primary>DISTINCT</primary>
    <secondary>SELECT DISTINCT</secondary>
   </indexterm>

   <indexterm zone="queries-distinct">
    <primary>duplicates</primary>
   </indexterm>

   <para>
    After the select list has been processed, the result table can