proof-reading

[postgresql] / doc / src / sgml / queries.sgml

<!-- $Header: /cvsroot/pgsql/doc/src/sgml/queries.sgml,v 1.14 2001/11/28 20:49:10 petere Exp $ -->

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

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

 <para>
  A <firstterm>query</firstterm> is the process of retrieving or the command
  to retrieve data from a database.  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.  The simplest kind of
  query has the form
<programlisting>
SELECT * FROM table1;
</programlisting>
  Assuming that there is a table called table1, this command would
  retrieve all rows and all columns from table1.  (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, 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 even make calculations on the
  columns before retrieving them; see <xref
  linkend="queries-select-lists">.  For example, if table1 has columns
  named a, b, and c (and perhaps others) you can make the following
  query:
<programlisting>
SELECT a, b + c FROM table1;
</programlisting>
  (assuming that b and c are of a numeric data type).
 </para>

 <para>
  <literal>FROM table1</literal> is a particularly simple kind of
  table expression.  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 SELECT 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> specifies a table.  The
   table expression contains a FROM clause that is optionally followed
   by WHERE, GROUP BY, and 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 WHERE, GROUP BY, and HAVING clauses in the table expression
   specify a pipeline of successive transformations performed on the
   table derived in the FROM clause.  The derived table that is produced by
   all these transformations provides the input rows used to compute output
   rows as specified by the select list of column value expressions.
  </para>
        
  <sect2 id="queries-from">
   <title>FROM clause</title>
 
   <para>
    The 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 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 FROM clause they are
    cross-joined (see below) to form the derived table that may then
    be subject to transformations by the WHERE, GROUP BY, and 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 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 JOIN are supported.
    </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 <token>INNER</token> and <token>OUTER</token> are
        optional for all joins.  <token>INNER</token> is the default;
        <token>LEFT</token>, <token>RIGHT</token>, and
        <token>FULL</token> imply an OUTER JOIN.
       </para>

       <para>
        The <firstterm>join condition</firstterm> is specified in the
        ON or USING clause, or implicitly by the word 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 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 WHERE
        clause.  A pair of rows from T1 and T2 match if the ON expression
        evaluates to TRUE for them.
       </para>

       <para>
        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 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 ON is used there will be two columns a, b, and c in the
        result, whereas with USING there will be only one of each.
       </para>

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

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

       <variablelist>
        <varlistentry>
         <term>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>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 returned 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>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 returned 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>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 returned 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 returned.
          </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 JOIN clauses to control the join order.  In the
     absence of parentheses, JOIN clauses nest left-to-right.
    </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 named using an AS
     clause.  (See <xref linkend="queries-table-aliases">.)
    </para>

<programlisting>
FROM (SELECT * FROM table1) AS alias_name
</programlisting>

    <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>

   <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>.
<synopsis>
FROM <replaceable>table_reference</replaceable> AS <replaceable>alias</replaceable>
</synopsis>
     Here, <replaceable>alias</replaceable> can be any regular
     identifier.  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 FROM 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>
     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.
    </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>
<synopsis>
FROM <replaceable>table_reference</replaceable> <replaceable>alias</replaceable>
</synopsis>
     This form is equivalent to the previously treated one; the
     <token>AS</token> key word is noise.
    </para>

    <para>
<synopsis>
FROM <replaceable>table_reference</replaceable> <optional>AS</optional> <replaceable>alias</replaceable> ( <replaceable>column1</replaceable> <optional>, <replaceable>column2</replaceable> <optional>, ...</optional></optional> )
</synopsis>
     In this form,
     in addition to renaming the table as described above, the columns
     of the table are also given temporary names for use by the surrounding
     query.  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 JOIN clause, using any of
     these forms, the alias hides the original names within the 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 A is not visible outside the alias C.
    </para>
   </sect3>

   <sect3 id="queries-table-expression-examples">
    <title>Examples</title>

    <para>
<programlisting>
FROM T1 INNER JOIN T2 USING (C)
FROM T1 LEFT OUTER JOIN T2 USING (C)
FROM (T1 RIGHT OUTER JOIN T2 ON (T1.C1=T2.C1)) AS DT1
FROM (T1 FULL OUTER JOIN T2 USING (C)) AS DT1 (DT1C1, DT1C2)

FROM T1 NATURAL INNER JOIN T2
FROM T1 NATURAL LEFT OUTER JOIN T2
FROM T1 NATURAL RIGHT OUTER JOIN T2
FROM T1 NATURAL FULL OUTER JOIN T2

FROM (SELECT * FROM T1) DT1 CROSS JOIN T2, T3
FROM (SELECT * FROM T1) DT1, T2, T3
</programlisting>

     Above are some examples of joined tables and complex derived
     tables.  Notice how the AS clause renames or names a derived
     table and how the optional comma-separated list of column names
     that follows renames the columns.  The last two
     FROM clauses produce the same derived table from T1, T2, and T3.
     The AS keyword was omitted in naming the subquery as DT1.  The
     keywords OUTER and INNER are noise that can be omitted also.
    </para>
   </sect3>

  </sect2>

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

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

   <para>
    The syntax of the 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 FROM clause is done, each row of the
    derived 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 FROM clause; this is not
    required, but otherwise the WHERE clause will be fairly useless.
   </para>

   <note>
    <para>
     Before the implementation of the JOIN syntax, it was necessary to
     put the join condition of an inner join in the 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 JOIN
     syntax in the FROM clause is probably not as portable to other
     products.  For outer joins there is no choice in any case:  they
     must be done in the FROM clause.  A ON/USING clause of an outer join
     is <emphasis>not</> equivalent to a 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>

<programlisting>
FROM FDT WHERE
    C1 > 5

FROM FDT WHERE
    C1 IN (1, 2, 3)
FROM FDT WHERE
    C1 IN (SELECT C1 FROM T2)
FROM FDT WHERE
    C1 IN (SELECT C3 FROM T2 WHERE C2 = FDT.C1 + 10)

FROM FDT WHERE
    C1 BETWEEN (SELECT C3 FROM T2 WHERE C2 = FDT.C1 + 10) AND 100

FROM FDT WHERE
    EXISTS (SELECT C1 FROM T2 WHERE C2 > FDT.C1)
</programlisting>

   <para>
    In the examples above, <literal>FDT</literal> is the table derived
    in the FROM clause. Rows that do not meet the search condition of
    the 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>GROUP BY and HAVING clauses</title>      

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

   <para>
    After passing the WHERE filter, the derived input table may be
    subject to grouping, using the GROUP BY clause, and elimination of
    group rows using the 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 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 (as opposed to an
    ORDER BY clause).  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 obtain aggregates that apply to these groups.
   </para>
          
   <para>
    Once a table is grouped, columns that are not used in the
    grouping cannot be referenced except in aggregate expressions,
    since a specific value in those columns is ambiguous - which row
    in the group should it come from?  The grouped-by columns can be
    referenced in select list column expressions since they have a
    known constant value per group.  Aggregate functions on the
    ungrouped columns provide values that span the rows of a group,
    not of the whole table.  For instance, a
    <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.  Aggregates computed on the ungrouped columns are
    representative of the group, whereas individual values of an ungrouped
    column are not.
   </para>

   <para>
    Example:
<programlisting>
SELECT pid, p.name, (sum(s.units) * p.price) AS sales
  FROM products p LEFT JOIN sales s USING ( pid )
  GROUP BY pid, p.name, p.price;
</programlisting>
    In this example, the columns <literal>pid</literal>, <literal>p.name</literal>, and <literal>p.price</literal> must be in
    the GROUP BY clause since they are referenced in the query select
    list.  The column s.units does not have to be in the 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, GROUP BY can only group by columns of the source
    table but <productname>PostgreSQL</productname> extends this to also allow GROUP BY to group by
    select columns in the query select list.  Grouping by value
    expressions instead of simple column names is also allowed.
   </para>

   <para>
<synopsis>
SELECT <replaceable>select_list</replaceable> FROM ... <optional>WHERE ...</optional> GROUP BY ... HAVING <replaceable>boolean_expression</replaceable>
</synopsis>
    If a table has been grouped using a GROUP BY clause, but then only
    certain groups are of interest, the HAVING clause can be used,
    much like a WHERE clause, to eliminate groups from a grouped
    table.  <productname>PostgreSQL</productname> allows a HAVING clause to be
    used without a GROUP BY, in which case it acts like another WHERE
    clause, but the point in using HAVING that way is not clear.  A good
    rule of thumb is that a HAVING condition should refer to the results
    of aggregate functions.  A restriction that does not involve an
    aggregate is more efficiently expressed in the WHERE clause.
   </para>

   <para>
    Example:
<programlisting>
SELECT pid    AS "Products",
       p.name AS "Over 5000",
       (sum(s.units) * (p.price - p.cost)) AS "Past Month Profit"
  FROM products p LEFT JOIN sales s USING ( pid )
  WHERE s.date > CURRENT_DATE - INTERVAL '4 weeks'
  GROUP BY pid, p.name, p.price, p.cost
    HAVING sum(p.price * s.units) > 5000;
</programlisting>
    In the example above, the WHERE clause is selecting rows by a
    column that is not grouped, while the HAVING clause
    restricts the output to groups with total gross sales over 5000.
   </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.  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 a, b, and c are either the actual names of the
   columns of tables referenced in the 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
   WHERE clause (unless grouping is used, in which case it is the same
   as in the HAVING clause).  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 FROM clause; they could be
   constant arithmetic expressions as well, for instance.
  </para>

  <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 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
    <token>DISTINCT</token> key word is written directly after the
    <token>SELECT</token> to enable this:
<synopsis>
SELECT DISTINCT <replaceable>select_list</replaceable> ...
</synopsis>
    (Instead of <token>DISTINCT</token> the word <token>ALL</token>
    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.  NULLs 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 DISTINCT filter.  (DISTINCT ON processing
    occurs after ORDER BY sorting.)
   </para>

   <para>
    The 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 GROUP BY and subselects in
    FROM the construct can be avoided, but it is very 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>
   <command>UNION</command> 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 DISTINCT,
   unless ALL is specified.
  </para>

  <para>
   <command>INTERSECT</command> 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 ALL is specified.
  </para>

  <para>
   <command>EXCEPT</command> returns all rows that are in the result
   of <replaceable>query1</replaceable> but not in the result of
   <replaceable>query2</replaceable>.  Again, duplicates are
   eliminated unless ALL is specified.
  </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 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 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 UNION, INTERSECT,
   or EXCEPT, and are not portable to other <acronym>DBMS</acronym>.
  </para>

  <para>
   Each column specification may be followed by an optional <token>ASC</token> or
   <token>DESC</token> to set the sort direction.  <token>ASC</token> is 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 specifications.
  </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>

<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>
   LIMIT allows you to retrieve just a portion of the rows that are
   generated by the rest of the query.  If a limit count is given, no
   more than that many rows will be returned.
   LIMIT ALL is the same as omitting a LIMIT clause.
  </para>

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

  <para>
   When using LIMIT, it is a good idea to use an 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
   ORDER BY.
  </para>

  <para>
   The query optimizer takes 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 LIMIT and
   OFFSET.  Thus, using different LIMIT/OFFSET values to select
   different subsets of a query result <emphasis>will give
   inconsistent results</emphasis> unless you enforce a predictable
   result ordering with 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 ORDER
   BY is used to constrain the order.
  </para>
 </sect1>

</chapter>