path: root/doc/src/sgml/queries.sgml

<!-- $Header: /cvsroot/pgsql/doc/src/sgml/queries.sgml,v 1.18.2.1 2002/11/10 12:45:42 petere Exp $ -->

<chapter id="queries">
 <title>Queries</title>

 <para>
  The previous chapters explained how to create tables, how to fill
  them with data, and how to manipulate that data.  Now we finally
  discuss how to retrieve the data out of the database.
 </para>


 <sect1 id="queries-overview">
  <title>Overview</title>

  <para>
   The process of retrieving or the command to retrieve data from a
   database is called a <firstterm>query</firstterm>.  In SQL the
   <command>SELECT</command> command is used to specify queries.  The
   general syntax of the <command>SELECT</command> command is
<synopsis>
SELECT <replaceable>select_list</replaceable> FROM <replaceable>table_expression</replaceable> <optional><replaceable>sort_specification</replaceable></optional>
</synopsis>
   The following sections describe the details of the select list, the
   table expression, and the sort specification.
  </para>

  <para>
   The simplest kind of query has the form
<programlisting>
SELECT * FROM table1;
</programlisting>
  Assuming that there is a table called <literal>table1</literal>,
  this command would retrieve all rows and all columns from
  <literal>table1</literal>.  (The method of retrieval depends on the
  client application.  For example, the
  <application>psql</application> program will display an ASCII-art
  table on the screen, while client libraries will offer functions to
  retrieve individual rows and columns.)  The select list
  specification <literal>*</literal> means all columns that the table
  expression happens to provide.  A select list can also select a
  subset of the available columns or make calculations using the
  columns.  For example, if
  <literal>table1</literal> has columns named <literal>a</>,
  <literal>b</>, and <literal>c</> (and perhaps others) you can make
  the following query:
<programlisting>
SELECT a, b + c FROM table1;
</programlisting>
  (assuming that <literal>b</> and <literal>c</> are of a numerical
  data type).
  See <xref linkend="queries-select-lists"> for more details.
 </para>

 <para>
  <literal>FROM table1</literal> is a particularly simple kind of
  table expression: it reads just one table.  In general, table
  expressions can be complex constructs of base tables, joins, and
  subqueries.  But you can also omit the table expression entirely and
  use the <command>SELECT</command> command as a calculator:
<programlisting>
SELECT 3 * 4;
</programlisting>
  This is more useful if the expressions in the select list return
  varying results.  For example, you could call a function this way:
<programlisting>
SELECT random();
</programlisting>
  </para>
 </sect1>


 <sect1 id="queries-table-expressions">
  <title>Table Expressions</title>

  <para>
   A <firstterm>table expression</firstterm> computes a table.  The
   table expression contains a <literal>FROM</> clause that is
   optionally followed by <literal>WHERE</>, <literal>GROUP BY</>, and
   <literal>HAVING</> clauses.  Trivial table expressions simply refer
   to a table on disk, a so-called base table, but more complex
   expressions can be used to modify or combine base tables in various
   ways.
  </para>

  <para>
   The optional <literal>WHERE</>, <literal>GROUP BY</>, and
   <literal>HAVING</> clauses in the table expression specify a
   pipeline of successive transformations performed on the table
   derived in the <literal>FROM</> clause.  All these transformations
   produce a virtual table that provides the rows that are passed to
   the select list to compute the output rows of the query.
  </para>
	
  <sect2 id="queries-from">
   <title>The FROM Clause</title>
 
   <para>
    The <literal>FROM</> clause derives a table from one or more other
    tables given in a comma-separated table reference list.
<synopsis>
FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_reference</replaceable> <optional>, ...</optional></optional>
</synopsis>

    A table reference may be a table name (possibly schema-qualified),
    or a derived table such as a subquery, a table join, or complex
    combinations of these.  If more than one table reference is listed
    in the <literal>FROM</> clause they are cross-joined (see below)
    to form the intermediate virtual table that may then be subject to
    transformations by the <literal>WHERE</>, <literal>GROUP BY</>,
    and <literal>HAVING</> clauses and is finally the result of the
    overall table expression.
   </para>

   <para>
    When a table reference names a table that is the supertable of a
    table inheritance hierarchy, the table reference produces rows of
    not only that table but all of its subtable successors, unless the
    keyword <literal>ONLY</> precedes the table name.  However, the
    reference produces only the columns that appear in the named table
    --- any columns added in subtables are ignored.
   </para>

   <sect3 id="queries-join">
    <title>Joined Tables</title>

    <indexterm zone="queries-join">
     <primary>joins</primary>
    </indexterm>

    <para>
     A joined table is a table derived from two other (real or
     derived) tables according to the rules of the particular join
     type.  Inner, outer, and cross-joins are available.
    </para>

    <variablelist>
     <title>Join Types</title>

     <varlistentry>
      <term>Cross join</term>

      <indexterm>
       <primary>joins</primary>
       <secondary>cross</secondary>
      </indexterm>

      <listitem>
<synopsis>
<replaceable>T1</replaceable> CROSS JOIN <replaceable>T2</replaceable>
</synopsis>

       <para>
        For each combination of rows from
        <replaceable>T1</replaceable> and
        <replaceable>T2</replaceable>, the derived table will contain a
        row consisting of all columns in <replaceable>T1</replaceable>
        followed by all columns in <replaceable>T2</replaceable>.  If
        the tables have N and M rows respectively, the joined
        table will have N * M rows.  A cross join is equivalent to an
        <literal>INNER JOIN ON TRUE</literal>.
       </para>

       <tip>
        <para>
         <literal>FROM <replaceable>T1</replaceable> CROSS JOIN
         <replaceable>T2</replaceable></literal> is equivalent to
         <literal>FROM <replaceable>T1</replaceable>,
         <replaceable>T2</replaceable></literal>.
        </para>
       </tip>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>Qualified joins</term>

      <indexterm>
       <primary>joins</primary>
       <secondary>outer</secondary>
      </indexterm>

      <listitem>
<synopsis>
<replaceable>T1</replaceable> { <optional>INNER</optional> | { LEFT | RIGHT | FULL } <optional>OUTER</optional> } JOIN <replaceable>T2</replaceable> ON <replaceable>boolean_expression</replaceable>
<replaceable>T1</replaceable> { <optional>INNER</optional> | { LEFT | RIGHT | FULL } <optional>OUTER</optional> } JOIN <replaceable>T2</replaceable> USING ( <replaceable>join column list</replaceable> )
<replaceable>T1</replaceable> NATURAL { <optional>INNER</optional> | { LEFT | RIGHT | FULL } <optional>OUTER</optional> } JOIN <replaceable>T2</replaceable>
</synopsis>
        
       <para>
        The words <literal>INNER</literal> and
        <literal>OUTER</literal> are optional in all forms.
        <literal>INNER</literal> is the default;
        <literal>LEFT</literal>, <literal>RIGHT</literal>, and
        <literal>FULL</literal> imply an outer join.
       </para>

       <para>
        The <firstterm>join condition</firstterm> is specified in the
        <literal>ON</> or <literal>USING</> clause, or implicitly by
        the word <literal>NATURAL</>.  The join condition determines
        which rows from the two source tables are considered to
        <quote>match</quote>, as explained in detail below.
       </para>

       <para>
        The <literal>ON</> clause is the most general kind of join
        condition: it takes a Boolean value expression of the same
        kind as is used in a <literal>WHERE</> clause.  A pair of rows
        from <replaceable>T1</> and <replaceable>T2</> match if the
        <literal>ON</> expression evaluates to true for them.
       </para>

       <para>
        <literal>USING</> is a shorthand notation: it takes a
        comma-separated list of column names, which the joined tables
        must have in common, and forms a join condition specifying
        equality of each of these pairs of columns.  Furthermore, the
        output of a <literal>JOIN USING</> has one column for each of
        the equated pairs of input columns, followed by all of the
        other columns from each table.  Thus, <literal>USING (a, b,
        c)</literal> is equivalent to <literal>ON (t1.a = t2.a AND
        t1.b = t2.b AND t1.c = t2.c)</literal> with the exception that
        if <literal>ON</> is used there will be two columns
        <literal>a</>, <literal>b</>, and <literal>c</> in the result,
        whereas with <literal>USING</> there will be only one of each.
       </para>

       <para>
        <indexterm>
	 <primary>joins</primary>
	 <secondary>natural</secondary>
	</indexterm>
        Finally, <literal>NATURAL</> is a shorthand form of
        <literal>USING</>: it forms a <literal>USING</> list
        consisting of exactly those column names that appear in both
        input tables.  As with <literal>USING</>, these columns appear
        only once in the output table.
       </para>

       <para>
        The possible types of qualified join are:
       </para>

       <variablelist>
        <varlistentry>
         <term><literal>INNER JOIN</></term>

         <listitem>
          <para>
           For each row R1 of T1, the joined table has a row for each
           row in T2 that satisfies the join condition with R1.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>LEFT OUTER JOIN</></term>

         <indexterm>
          <primary>joins</primary>
          <secondary>left</secondary>
         </indexterm>

         <listitem>
          <para>
           First, an inner join is performed.  Then, for each row in
           T1 that does not satisfy the join condition with any row in
           T2, a joined row is added with null values in columns of
           T2.  Thus, the joined table unconditionally has at least
           one row for each row in T1.
          </para>
         </listitem>
        </varlistentry>
         
        <varlistentry>
         <term><literal>RIGHT OUTER JOIN</></term>

         <listitem>
          <para>
           First, an inner join is performed.  Then, for each row in
           T2 that does not satisfy the join condition with any row in
           T1, a joined row is added with null values in columns of
           T1.  This is the converse of a left join: the result table
           will unconditionally have a row for each row in T2.
          </para>
         </listitem>
        </varlistentry>
         
        <varlistentry>
         <term><literal>FULL OUTER JOIN</></term>

         <listitem>
          <para>
           First, an inner join is performed.  Then, for each row in
           T1 that does not satisfy the join condition with any row in
           T2, a joined row is added with null values in columns of
           T2.  Also, for each row of T2 that does not satisfy the
           join condition with any row in T1, a joined row with null
           values in the columns of T1 is added.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>
      </listitem>
     </varlistentry>
    </variablelist>

    <para>
     Joins of all types can be chained together or nested: either or
     both of <replaceable>T1</replaceable> and
     <replaceable>T2</replaceable> may be joined tables.  Parentheses
     may be used around <literal>JOIN</> clauses to control the join
     order.  In the absence of parentheses, <literal>JOIN</> clauses
     nest left-to-right.
    </para>

    <para>
     To put this together, assume we have tables <literal>t1</literal>
<programlisting>
 num | name
-----+------
   1 | a
   2 | b
   3 | c
</programlisting>
     and <literal>t2</literal>
<programlisting>
 num | value
-----+-------
   1 | xxx
   3 | yyy
   5 | zzz
</programlisting>
     then we get the following results for the various joins:
<screen>
<prompt>=></> <userinput>SELECT * FROM t1 CROSS JOIN t2;</>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   1 | a    |   3 | yyy
   1 | a    |   5 | zzz
   2 | b    |   1 | xxx
   2 | b    |   3 | yyy
   2 | b    |   5 | zzz
   3 | c    |   1 | xxx
   3 | c    |   3 | yyy
   3 | c    |   5 | zzz
(9 rows)

<prompt>=></> <userinput>SELECT * FROM t1 INNER JOIN t2 ON t1.num = t2.num;</>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   3 | c    |   3 | yyy
(2 rows)

<prompt>=></> <userinput>SELECT * FROM t1 INNER JOIN t2 USING (num);</>
 num | name | value
-----+------+-------
   1 | a    | xxx
   3 | c    | yyy
(2 rows)

<prompt>=></> <userinput>SELECT * FROM t1 NATURAL INNER JOIN t2;</>
 num | name | value
-----+------+-------
   1 | a    | xxx
   3 | c    | yyy
(2 rows)

<prompt>=></> <userinput>SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num;</>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   2 | b    |     |
   3 | c    |   3 | yyy
(3 rows)

<prompt>=></> <userinput>SELECT * FROM t1 LEFT JOIN t2 USING (num);</>
 num | name | value
-----+------+-------
   1 | a    | xxx
   2 | b    |
   3 | c    | yyy
(3 rows)

<prompt>=></> <userinput>SELECT * FROM t1 RIGHT JOIN t2 ON t1.num = t2.num;</>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   3 | c    |   3 | yyy
     |      |   5 | zzz
(3 rows)

<prompt>=></> <userinput>SELECT * FROM t1 FULL JOIN t2 ON t1.num = t2.num;</>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   2 | b    |     |
   3 | c    |   3 | yyy
     |      |   5 | zzz
(4 rows)
</screen>
    </para>

    <para>
     The join condition specified with <literal>ON</> can also contain
     conditions that do not relate directly to the join.  This can
     prove useful for some queries but needs to be thought out
     carefully.  For example:
<screen>
<prompt>=></> <userinput>SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num AND t2.value = 'xxx';</>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   2 | b    |     |
   3 | c    |     |
(3 rows)
</screen>
    </para>
   </sect3>

   <sect3 id="queries-table-aliases">
    <title>Table and Column Aliases</title>

    <indexterm zone="queries-table-aliases">
     <primary>label</primary>
     <secondary>table</secondary>
    </indexterm>

    <indexterm>
     <primary>alias</primary>
     <see>label</see>
    </indexterm>

    <para>
     A temporary name can be given to tables and complex table
     references to be used for references to the derived table in
     further processing.  This is called a <firstterm>table
     alias</firstterm>.
    </para>

    <para>
     To create a table alias, write
<synopsis>
FROM <replaceable>table_reference</replaceable> AS <replaceable>alias</replaceable>
</synopsis>
     or
<synopsis>
FROM <replaceable>table_reference</replaceable> <replaceable>alias</replaceable>
</synopsis>
     The <literal>AS</literal> key word is noise.
     <replaceable>alias</replaceable> can be any identifier.
    </para>

    <para>
     A typical application of table aliases is to assign short
     identifiers to long table names to keep the join clauses
     readable.  For example:
<programlisting>
SELECT * FROM some_very_long_table_name s JOIN another_fairly_long_name a ON s.id = a.num;
</programlisting>
    </para>

    <para>
     The alias becomes the new name of the table reference for the
     current query -- it is no longer possible to refer to the table
     by the original name.  Thus
<programlisting>
SELECT * FROM my_table AS m WHERE my_table.a > 5;
</programlisting>
     is not valid SQL syntax.  What will actually happen (this is a
     <productname>PostgreSQL</productname> extension to the standard)
     is that an implicit table reference is added to the
     <literal>FROM</literal> clause, so the query is processed as if
     it were written as
<programlisting>
SELECT * FROM my_table AS m, my_table AS my_table WHERE my_table.a > 5;
</programlisting>
     which will result in a cross join, which is usually not what you
     want.
    </para>

    <para>
     Table aliases are mainly for notational convenience, but it is
     necessary to use them when joining a table to itself, e.g.,
<programlisting>
SELECT * FROM my_table AS a CROSS JOIN my_table AS b ...
</programlisting>
     Additionally, an alias is required if the table reference is a
     subquery (see <xref linkend="queries-subqueries">).
    </para>

    <para>
     Parentheses are used to resolve ambiguities.  The following
     statement will assign the alias <literal>b</literal> to the
     result of the join, unlike the previous example:
<programlisting>
SELECT * FROM (my_table AS a CROSS JOIN my_table) AS b ...
</programlisting>
    </para>

    <para>
     Another form of table aliasing also gives temporary names to the columns of the table:
<synopsis>
FROM <replaceable>table_reference</replaceable> <optional>AS</optional> <replaceable>alias</replaceable> ( <replaceable>column1</replaceable> <optional>, <replaceable>column2</replaceable> <optional>, ...</optional></optional> )
</synopsis>
     If fewer column aliases are specified than the actual table has
     columns, the remaining columns are not renamed.  This syntax is
     especially useful for self-joins or subqueries.
    </para>

    <para>
     When an alias is applied to the output of a <literal>JOIN</>
     clause, using any of these forms, the alias hides the original
     names within the <literal>JOIN</>.  For example,
<programlisting>
SELECT a.* FROM my_table AS a JOIN your_table AS b ON ...
</programlisting>
     is valid SQL, but
<programlisting>
SELECT a.* FROM (my_table AS a JOIN your_table AS b ON ...) AS c
</programlisting>
     is not valid: the table alias <literal>a</> is not visible
     outside the alias <literal>c</>.
    </para>
   </sect3>

   <sect3 id="queries-subqueries">
    <title>Subqueries</title>

    <indexterm zone="queries-subqueries">
     <primary>subqueries</primary>
    </indexterm>

    <para>
     Subqueries specifying a derived table must be enclosed in
     parentheses and <emphasis>must</emphasis> be assigned a table
     alias name.  (See <xref linkend="queries-table-aliases">.)  For
     example:
<programlisting>
FROM (SELECT * FROM table1) AS alias_name
</programlisting>
    </para>

    <para>
     This example is equivalent to <literal>FROM table1 AS
     alias_name</literal>.  More interesting cases, which can't be
     reduced to a plain join, arise when the subquery involves
     grouping or aggregation.
    </para>
   </sect3>
  </sect2>

  <sect2 id="queries-where">
   <title>The WHERE Clause</title>

   <indexterm zone="queries-where">
    <primary>where</primary>
   </indexterm>

   <para>
    The syntax of the <literal>WHERE</> clause is
<synopsis>
WHERE <replaceable>search_condition</replaceable>
</synopsis>
    where <replaceable>search_condition</replaceable> is any value
    expression as defined in <xref linkend="sql-expressions"> that
    returns a value of type <type>boolean</type>.
   </para>

   <para>
    After the processing of the <literal>FROM</> clause is done, each
    row of the derived virtual table is checked against the search
    condition.  If the result of the condition is true, the row is
    kept in the output table, otherwise (that is, if the result is
    false or null) it is discarded.  The search condition typically
    references at least some column in the table generated in the
    <literal>FROM</> clause; this is not required, but otherwise the
    <literal>WHERE</> clause will be fairly useless.
   </para>

   <note>
    <para>
     Before the implementation of the <literal>JOIN</> syntax, it was
     necessary to put the join condition of an inner join in the
     <literal>WHERE</> clause.  For example, these table expressions
     are equivalent:
<programlisting>
FROM a, b WHERE a.id = b.id AND b.val &gt; 5
</programlisting>
     and
<programlisting>
FROM a INNER JOIN b ON (a.id = b.id) WHERE b.val &gt; 5
</programlisting>
     or perhaps even
<programlisting>
FROM a NATURAL JOIN b WHERE b.val &gt; 5
</programlisting>
     Which one of these you use is mainly a matter of style.  The
     <literal>JOIN</> syntax in the <literal>FROM</> clause is
     probably not as portable to other SQL database products.  For
     outer joins there is no choice in any case: they must be done in
     the <literal>FROM</> clause.  An <literal>ON</>/<literal>USING</>
     clause of an outer join is <emphasis>not</> equivalent to a
     <literal>WHERE</> condition, because it determines the addition
     of rows (for unmatched input rows) as well as the removal of rows
     from the final result.
    </para>
   </note>

   <para>
    Here are some examples of <literal>WHERE</literal> clauses:
<programlisting>
SELECT ... FROM fdt WHERE c1 > 5

SELECT ... FROM fdt WHERE c1 IN (1, 2, 3)

SELECT ... FROM fdt WHERE c1 IN (SELECT c1 FROM t2)

SELECT ... FROM fdt WHERE c1 IN (SELECT c3 FROM t2 WHERE c2 = fdt.c1 + 10)

SELECT ... FROM fdt WHERE c1 BETWEEN (SELECT c3 FROM t2 WHERE c2 = fdt.c1 + 10) AND 100

SELECT ... FROM fdt WHERE EXISTS (SELECT c1 FROM t2 WHERE c2 > fdt.c1)
</programlisting>
    <literal>fdt</literal> is the table derived in the
    <literal>FROM</> clause. Rows that do not meet the search
    condition of the <literal>WHERE</> clause are eliminated from
    <literal>fdt</literal>. Notice the use of scalar subqueries as
    value expressions.  Just like any other query, the subqueries can
    employ complex table expressions.  Notice how
    <literal>fdt</literal> is referenced in the subqueries.
    Qualifying <literal>c1</> as <literal>fdt.c1</> is only necessary
    if <literal>c1</> is also the name of a column in the derived
    input table of the subquery.  Qualifying the column name adds
    clarity even when it is not needed.  This shows how the column
    naming scope of an outer query extends into its inner queries.
   </para>
  </sect2>


  <sect2 id="queries-group">
   <title>The GROUP BY and HAVING Clauses</title>

   <indexterm zone="queries-group">
    <primary>group</primary>
   </indexterm>

   <para>
    After passing the <literal>WHERE</> filter, the derived input
    table may be subject to grouping, using the <literal>GROUP BY</>
    clause, and elimination of group rows using the <literal>HAVING</>
    clause.
   </para>

<synopsis>
SELECT <replaceable>select_list</replaceable>
    FROM ...
    <optional>WHERE ...</optional>
    GROUP BY <replaceable>grouping_column_reference</replaceable> <optional>, <replaceable>grouping_column_reference</replaceable></optional>...
</synopsis>

   <para>
    The <literal>GROUP BY</> clause is used to group together rows in
    a table that share the same values in all the columns listed. The
    order in which the columns are listed does not matter.  The
    purpose is to reduce each group of rows sharing common values into
    one group row that is representative of all rows in the group.
    This is done to eliminate redundancy in the output and/or compute
    aggregates that apply to these groups.  For instance:
<screen>
<prompt>=></> <userinput>SELECT * FROM test1;</>
 x | y
---+---
 a | 3
 c | 2
 b | 5
 a | 1
(4 rows)

<prompt>=></> <userinput>SELECT x FROM test1 GROUP BY x;</>
 x
---
 a
 b
 c
(3 rows)
</screen>
   </para>
	  
   <para>
    In the second query, we could not have written <literal>SELECT *
    FROM test1 GROUP BY x</literal>, because there is no single value
    for the column <literal>y</> that could be associated with each
    group.  The grouped-by columns can be referenced in the select list since
    they have a known constant value per group.
   </para>

   <para>
    In general, if a table is grouped, columns that are not
    used in the grouping cannot be referenced except in aggregate
    expressions.  An example with aggregate expressions is:
<screen>
<prompt>=></> <userinput>SELECT x, sum(y) FROM test1 GROUP BY x;</>
 x | sum
---+-----
 a |   4
 b |   5
 c |   2
(3 rows)
</screen>
    Here <literal>sum()</literal> is an aggregate function that
    computes a single value over the entire group.  More information
    about the available aggregate functions can be found in <xref
    linkend="functions-aggregate">.
   </para>

   <tip>
    <para>
     Grouping without aggregate expressions effectively calculates the
     set of distinct values in a column.  This can also be achieved
     using the <literal>DISTINCT</> clause (see <xref
     linkend="queries-distinct">).
    </para>
   </tip>

   <para>
    Here is another example:  <function>sum(sales)</function> on a
    table grouped by product code gives the total sales for each
    product, not the total sales on all products.
<programlisting>
SELECT product_id, p.name, (sum(s.units) * p.price) AS sales
    FROM products p LEFT JOIN sales s USING (product_id)
    GROUP BY product_id, p.name, p.price;
</programlisting>
    In this example, the columns <literal>product_id</literal>,
    <literal>p.name</literal>, and <literal>p.price</literal> must be
    in the <literal>GROUP BY</> clause since they are referenced in
    the query select list.  (Depending on how exactly the products
    table is set up, name and price may be fully dependent on the
    product ID, so the additional groupings could theoretically be
    unnecessary, but this is not implemented yet.)  The column
    <literal>s.units</> does not have to be in the <literal>GROUP
    BY</> list since it is only used in an aggregate expression
    (<function>sum()</function>), which represents the group of sales
    of a product.  For each product, a summary row is returned about
    all sales of the product.
   </para>

   <para>
    In strict SQL, <literal>GROUP BY</> can only group by columns of
    the source table but <productname>PostgreSQL</productname> extends
    this to also allow <literal>GROUP BY</> to group by columns in the
    select list.  Grouping by value expressions instead of simple
    column names is also allowed.
   </para>

   <para>
    If a table has been grouped using a <literal>GROUP BY</literal>
    clause, but then only certain groups are of interest, the
    <literal>HAVING</literal> clause can be used, much like a
    <literal>WHERE</> clause, to eliminate groups from a grouped
    table.  The syntax is:
<synopsis>
SELECT <replaceable>select_list</replaceable> FROM ... <optional>WHERE ...</optional> GROUP BY ... HAVING <replaceable>boolean_expression</replaceable>
</synopsis>
    Expressions in the <literal>HAVING</> clause can refer both to
    grouped expressions and to ungrouped expressions (which necessarily
    involve an aggregate function).
   </para>

   <para>
    Example:
<screen>
<prompt>=></> <userinput>SELECT x, sum(y) FROM test1 GROUP BY x HAVING sum(y) > 3;</>
 x | sum
---+-----
 a |   4
 b |   5
(2 rows)

<prompt>=></> <userinput>SELECT x, sum(y) FROM test1 GROUP BY x HAVING x < 'c';</>
 x | sum
---+-----
 a |   4
 b |   5
(2 rows)
</screen>
   </para>

   <para>
    Again, a more realistic example:
<programlisting>
SELECT product_id, p.name, (sum(s.units) * (p.price - p.cost)) AS profit
    FROM products p LEFT JOIN sales s USING (product_id)
    WHERE s.date > CURRENT_DATE - INTERVAL '4 weeks'
    GROUP BY product_id, p.name, p.price, p.cost
    HAVING sum(p.price * s.units) > 5000;
</programlisting>
    In the example above, the <literal>WHERE</> clause is selecting
    rows by a column that is not grouped, while the <literal>HAVING</>
    clause restricts the output to groups with total gross sales over
    5000.  Note that the aggregate expressions do not necessarily need
    to be the same everywhere.
   </para>
  </sect2>
 </sect1>


 <sect1 id="queries-select-lists">
  <title>Select Lists</title>

  <indexterm>
   <primary>select</primary>
   <secondary>select list</secondary>
  </indexterm>

  <para>
   As shown in the previous section,
   the table expression in the <command>SELECT</command> command
   constructs an intermediate virtual table by possibly combining
   tables, views, eliminating rows, grouping, etc.  This table is
   finally passed on to processing by the <firstterm>select list</firstterm>.  The select
   list determines which <emphasis>columns</emphasis> of the
   intermediate table are actually output.
  </para>

  <sect2 id="queries-select-list-items">
   <title>Select-List Items</title>

   <para>
    The simplest kind of select list is <literal>*</literal> which
    emits all columns that the table expression produces.  Otherwise,
    a select list is a comma-separated list of value expressions (as
    defined in <xref linkend="sql-expressions">).  For instance, it
    could be a list of column names:
<programlisting>
SELECT a, b, c FROM ...
</programlisting>
     The columns names <literal>a</>, <literal>b</>, and <literal>c</>
     are either the actual names of the columns of tables referenced
     in the <literal>FROM</> 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</> clause, unless grouping is used, in which case
     it is the same as in the <literal>HAVING</> 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.b, tbl1.c FROM ...
</programlisting>
    (See also <xref linkend="queries-where">.)
   </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 retrieved 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</> clause;
    they could be constant arithmetic expressions as well, for
    instance.
   </para>
  </sect2>

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

   <indexterm zone="queries-column-labels">
    <primary>label</primary>
    <secondary>column</secondary>
   </indexterm>

   <para>
    The entries in the select list can be assigned names for further
    processing.  The <quote>further processing</quote> in this case is
    an optional sort specification and the client application (e.g.,
    column headers for display).  For example:
<programlisting>
SELECT a AS value, b + c AS sum FROM ...
</programlisting>
   </para>

   <para>
    If no output column name is specified via AS, the system assigns a
    default 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>

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

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

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

   <para>
    After the select list has been processed, the result table may
    optionally be subject to the elimination of duplicates.  The
    <literal>DISTINCT</literal> key word is written directly after the
    <literal>SELECT</literal> to enable this:
<synopsis>
SELECT DISTINCT <replaceable>select_list</replaceable> ...
</synopsis>
    (Instead of <literal>DISTINCT</> the word <literal>ALL</literal>
    can be used to select the default behavior of retaining all rows.)
   </para>

   <para>
    Obviously, two rows are considered distinct if they differ in at
    least one column value.  Null values are considered equal in this
    comparison.
   </para>

   <para>
    Alternatively, an arbitrary expression can determine what rows are
    to be considered distinct:
<synopsis>
SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceable>expression</replaceable> ...</optional>) <replaceable>select_list</replaceable> ...
</synopsis>
    Here <replaceable>expression</replaceable> is an arbitrary value
    expression that is evaluated for all rows.  A set of rows for
    which all the expressions are equal are considered duplicates, and
    only the first row of the set is kept in the output.  Note that
    the <quote>first row</quote> of a set is unpredictable unless the
    query is sorted on enough columns to guarantee a unique ordering
    of the rows arriving at the <literal>DISTINCT</> filter.
    (<literal>DISTINCT ON</> processing occurs after <literal>ORDER
    BY</> sorting.)
   </para>

   <para>
    The <literal>DISTINCT ON</> clause is not part of the SQL standard
    and is sometimes considered bad style because of the potentially
    indeterminate nature of its results.  With judicious use of
    <literal>GROUP BY</> and subselects in <literal>FROM</> the
    construct can be avoided, but it is often the most convenient
    alternative.
   </para>
  </sect2>
 </sect1>


 <sect1 id="queries-union">
  <title>Combining Queries</title>

  <indexterm zone="queries-union">
   <primary>union</primary>
  </indexterm>
  <indexterm zone="queries-union">
   <primary>intersection</primary>
  </indexterm>
  <indexterm zone="queries-union">
   <primary>except</primary>
  </indexterm>

  <para>
   The results of two queries can be combined using the set operations
   union, intersection, and difference.  The syntax is
<synopsis>
<replaceable>query1</replaceable> UNION <optional>ALL</optional> <replaceable>query2</replaceable>
<replaceable>query1</replaceable> INTERSECT <optional>ALL</optional> <replaceable>query2</replaceable>
<replaceable>query1</replaceable> EXCEPT <optional>ALL</optional> <replaceable>query2</replaceable>
</synopsis>
   <replaceable>query1</replaceable> and
   <replaceable>query2</replaceable> are queries that can use any of
   the features discussed up to this point.  Set operations can also
   be nested and chained, for example
<synopsis>
<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable> UNION <replaceable>query3</replaceable>
</synopsis>
   which really says
<synopsis>
(<replaceable>query1</replaceable> UNION <replaceable>query2</replaceable>) UNION <replaceable>query3</replaceable>
</synopsis>
  </para>

  <para>
   <literal>UNION</> effectively appends the result of
   <replaceable>query2</replaceable> to the result of
   <replaceable>query1</replaceable> (although there is no guarantee
   that this is the order in which the rows are actually returned).
   Furthermore, it eliminates all duplicate rows, in the sense of
   <literal>DISTINCT</>, unless <literal>UNION ALL</> is used.
  </para>

  <para>
   <literal>INTERSECT</> returns all rows that are both in the result
   of <replaceable>query1</replaceable> and in the result of
   <replaceable>query2</replaceable>.  Duplicate rows are eliminated
   unless <literal>INTERSECT ALL</> is used.
  </para>

  <para>
   <literal>EXCEPT</> returns all rows that are in the result of
   <replaceable>query1</replaceable> but not in the result of
   <replaceable>query2</replaceable>.  (This is sometimes called the
   <firstterm>difference</> between two queries.)  Again, duplicates
   are eliminated unless <literal>EXCEPT ALL</> is used.
  </para>

  <para>
   In order to calculate the union, intersection, or difference of two
   queries, the two queries must be <quote>union compatible</quote>,
   which means that they both return the same number of columns, and
   that the corresponding columns have compatible data types, as
   described in <xref linkend="typeconv-union-case">.
  </para>
 </sect1>


 <sect1 id="queries-order">
  <title>Sorting Rows</title>

  <indexterm zone="queries-order">
   <primary>sorting</primary>
   <secondary>query results</secondary>
  </indexterm>
	   
  <para>
   After a query has produced an output table (after the select list
   has been processed) it can optionally be sorted.  If sorting is not
   chosen, the rows will be returned in random order.  The actual
   order in that case will depend on the scan and join plan types and
   the order on disk, but it must not be relied on.  A particular
   output ordering can only be guaranteed if the sort step is explicitly
   chosen.
  </para>

  <para>
   The <literal>ORDER BY</> clause specifies the sort order:
<synopsis>
SELECT <replaceable>select_list</replaceable>
    FROM <replaceable>table_expression</replaceable>
    ORDER BY <replaceable>column1</replaceable> <optional>ASC | DESC</optional> <optional>, <replaceable>column2</replaceable> <optional>ASC | DESC</optional> ...</optional>
</synopsis>
   <replaceable>column1</replaceable>, etc., refer to select list
   columns.  These can be either the output name of a column (see
   <xref linkend="queries-column-labels">) or the number of a column.  Some
   examples:
<programlisting>
SELECT a, b FROM table1 ORDER BY a;
SELECT a + b AS sum, c FROM table1 ORDER BY sum;
SELECT a, sum(b) FROM table1 GROUP BY a ORDER BY 1;
</programlisting>
  </para>

  <para>
   As an extension to the SQL standard, <productname>PostgreSQL</productname> also allows ordering
   by arbitrary expressions:
<programlisting>
SELECT a, b FROM table1 ORDER BY a + b;
</programlisting>
   References to column names in the <literal>FROM</> clause that are
   renamed in the select list are also allowed:
<programlisting>
SELECT a AS b FROM table1 ORDER BY a;
</programlisting>
   But these extensions do not work in queries involving
   <literal>UNION</>, <literal>INTERSECT</>, or <literal>EXCEPT</>,
   and are not portable to other SQL databases.
  </para>

  <para>
   Each column specification may be followed by an optional
   <literal>ASC</> or <literal>DESC</> to set the sort direction to
   ascending or descending.  <literal>ASC</> order is the default.
   Ascending order puts smaller values first, where
   <quote>smaller</quote> is defined in terms of the
   <literal>&lt;</literal> operator.  Similarly, descending order is
   determined with the <literal>&gt;</literal> operator.
  </para>

  <para>
   If more than one sort column is specified, the later entries are
   used to sort rows that are equal under the order imposed by the
   earlier sort columns.
  </para>
 </sect1>


 <sect1 id="queries-limit">
  <title>LIMIT and OFFSET</title>

  <indexterm zone="queries-limit">
   <primary>limit</primary>
  </indexterm>

  <indexterm zone="queries-limit">
   <primary>offset</primary>
   <secondary>with query results</secondary>
  </indexterm>

  <para>
   <literal>LIMIT</> and <literal>OFFSET</> allow you to retrieve just
   a portion of the rows that are generated by the rest of the query:
<synopsis>
SELECT <replaceable>select_list</replaceable>
    FROM <replaceable>table_expression</replaceable>
    <optional>LIMIT { <replaceable>number</replaceable> | ALL }</optional> <optional>OFFSET <replaceable>number</replaceable></optional>
</synopsis>
  </para>

  <para>
   If a limit count is given, no more than that many rows will be
   returned (but possibly less, if the query itself yields less rows).
   <literal>LIMIT ALL</> is the same as omitting the <literal>LIMIT</>
   clause.
  </para>

  <para>
   <literal>OFFSET</> says to skip that many rows before beginning to
   return rows to the client.  <literal>OFFSET 0</> is the same as
   omitting the <literal>OFFSET</> clause.  If both <literal>OFFSET</>
   and <literal>LIMIT</> appear, then <literal>OFFSET</> rows are
   skipped before starting to count the <literal>LIMIT</> rows that
   are returned.
  </para>

  <para>
   When using <literal>LIMIT</>, it is a good idea to use an
   <literal>ORDER BY</> clause that constrains the result rows into a
   unique order.  Otherwise you will get an unpredictable subset of
   the query's rows---you may be asking for the tenth through
   twentieth rows, but tenth through twentieth in what ordering?  The
   ordering is unknown, unless you specified <literal>ORDER BY</>.
  </para>

  <para>
   The query optimizer takes <literal>LIMIT</> into account when
   generating a query plan, so you are very likely to get different
   plans (yielding different row orders) depending on what you give
   for <literal>LIMIT</> and <literal>OFFSET</>.  Thus, using
   different <literal>LIMIT</>/<literal>OFFSET</> values to select
   different subsets of a query result <emphasis>will give
   inconsistent results</emphasis> unless you enforce a predictable
   result ordering with <literal>ORDER BY</>.  This is not a bug; it
   is an inherent consequence of the fact that SQL does not promise to
   deliver the results of a query in any particular order unless
   <literal>ORDER BY</> is used to constrain the order.
  </para>
 </sect1>

</chapter>