select.sgml

关系型数据库 Postgresql 6.5.2

    assign a name
    to a calculated column using the AS clause, e.g.:
    <programlisting>
SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;
    </programlisting></para>
    
   <para>
    From release 6.4 of PostgreSQL, the columns in the ORDER BY clause do not need to appear in the SELECT clause.
    Thus the following statement is now legal:
    <programlisting>
SELECT name FROM distributors ORDER BY code;
    </programlisting></para>
    
   <para>
    Optionally one may add the keyword DESC (descending)
    or ASC (ascending) after each column name in the ORDER BY clause.
    If not specified, ASC is assumed by default.</para>
  </refsect2>
  
  <refsect2 id="R2-SQL-UNION-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    UNION Clause
   </title>
   <para>
    <synopsis>
<replaceable class="PARAMETER">table_query</replaceable> UNION [ ALL ]
<replaceable class="PARAMETER">table_query</replaceable>
     [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
    </synopsis>
    
    where
    <replaceable class="PARAMETER">table_query</replaceable>
    specifies any select expression without an ORDER BY clause.</para>
    
   <para>
    The UNION clause allows the result to be the collection of rows
    returned by the queries involved. (See UNION clause).
    The two tables that represent the direct operands of the UNION must
    have the same number of columns, and corresponding columns must be
    of compatible data types.</para>
    
   <para>
    By default, the result of UNION does not contain any duplicate rows
    unless the ALL clause is specified.</para>
    
   <para>
    Multiple UNION operators in the same SELECT statement are
    evaluated left to right.
    Note that the ALL keyword is not global in nature, being 
    applied only for the current pair of table results.</para>
    
  </refsect2>

  <refsect2 id="R2-SQL-INTERSECT-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    INTERSECT Clause
   </title>
   <para>
    <synopsis>
<replaceable class="PARAMETER">table_query</replaceable> INTERSECT
<replaceable class="PARAMETER">table_query</replaceable>
     [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
    </synopsis>
    
    where
    <replaceable class="PARAMETER">table_query</replaceable>
    specifies any select expression without an ORDER BY clause.</para>
    
   <para>
    The INTERSECT clause allows the result to be all rows that are 
    common to the involved queries.  (See INTERSECT clause).
    The two tables that represent the direct operands of the INTERSECT must
    have the same number of columns, and corresponding columns must be
    of compatible data types.</para>
    
   <para>
    Multiple INTERSECT operators in the same SELECT statement are
    evaluated left to right.
    </para>
    
  </refsect2>

  <refsect2 id="R2-SQL-EXCEPT-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    EXCEPT Clause
   </title>
   <para>
    <synopsis>
<replaceable class="PARAMETER">table_query</replaceable> EXCEPT
     <replaceable class="PARAMETER">table_query</replaceable>
     [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
    </synopsis>
    
    where
    <replaceable class="PARAMETER">table_query</replaceable>
    specifies any select expression without an ORDER BY clause.</para>
    
   <para>
    The EXCEPT clause allows the result to be rows from the upper query
    that are not in the lower query.  (See EXCEPT clause).
    The two tables that represent the direct operands of the EXCEPT must
    have the same number of columns, and corresponding columns must be
    of compatible data types.</para>
    
   <para>
    Multiple EXCEPT operators in the same SELECT statement are
    evaluated left to right.
    </para>
    
  </refsect2>

</refsect1>
  
 <refsect1 id="R1-SQL-SELECT-2">
  <title>
   Usage
  </title>
  <para>
   To join the table <literal>films</literal> with the table
   <literal>distributors</literal>:
  </para>
  <programlisting>
SELECT f.title, f.did, d.name, f.date_prod, f.kind
    FROM distributors d, films f
    WHERE f.did = d.did

title                    |did|name            | date_prod|kind
-------------------------+---+----------------+----------+----------
The Third Man            |101|British Lion    |1949-12-23|Drama
The African Queen        |101|British Lion    |1951-08-11|Romantic
Une Femme est une Femme  |102|Jean Luc Godard |1961-03-12|Romantic
Vertigo                  |103|Paramount       |1958-11-14|Action
Becket                   |103|Paramount       |1964-02-03|Drama
48 Hrs                   |103|Paramount       |1982-10-22|Action
War and Peace            |104|Mosfilm         |1967-02-12|Drama
West Side Story          |105|United Artists  |1961-01-03|Musical
Bananas                  |105|United Artists  |1971-07-13|Comedy
Yojimbo                  |106|Toho            |1961-06-16|Drama
There's a Girl in my Soup|107|Columbia        |1970-06-11|Comedy
Taxi Driver              |107|Columbia        |1975-05-15|Action
Absence of Malice        |107|Columbia        |1981-11-15|Action
Storia di una donna      |108|Westward        |1970-08-15|Romantic
The King and I           |109|20th Century Fox|1956-08-11|Musical
Das Boot                 |110|Bavaria Atelier |1981-11-11|Drama
Bed Knobs and Broomsticks|111|Walt Disney     |          |Musical
  </programlisting>
  <para>
   To sum the column <literal>len</literal> of all films and group
   the results by <literal>kind</literal>:
  </para>
  <programlisting>
SELECT kind, SUM(len) AS total FROM films GROUP BY kind;

    kind      |total
    ----------+------
    Action    | 07:34
    Comedy    | 02:58
    Drama     | 14:28
    Musical   | 06:42
    Romantic  | 04:38
  </programlisting>

  <para>
   To sum the column <literal>len</literal> of all films, group
   the results by <literal>kind</literal> and show those group totals
   that are less than 5 hours:
  </para>
  <programlisting>
SELECT kind, SUM(len) AS total
    FROM films
    GROUP BY kind
    HAVING SUM(len) < INTERVAL '5 hour';

    kind      |total
    ----------+------
    Comedy    | 02:58
    Romantic  | 04:38
  </programlisting>
  <para>
    The following two examples are identical ways of sorting the individual
   results according to the contents of the second column
   (<literal>name</literal>):
  </para>
  <programlisting>
SELECT * FROM distributors ORDER BY name;
SELECT * FROM distributors ORDER BY 2;

    did|name
    ---+----------------
    109|20th Century Fox
    110|Bavaria Atelier
    101|British Lion
    107|Columbia
    102|Jean Luc Godard
    113|Luso films
    104|Mosfilm
    103|Paramount
    106|Toho
    105|United Artists
    111|Walt Disney
    112|Warner Bros.
    108|Westward
  </programlisting>

  <para>
   This example shows how to obtain the union of the tables
   <literal>distributors</literal> and
   <literal>actors</literal>, restricting the results to those that begin
   with letter W in each table.  Only distinct rows are to be used, so the
   ALL keyword is omitted:
  </para>
  <programlisting>
    --        distributors:                actors:
    --        did|name                     id|name
    --        ---+------------             --+--------------
    --        108|Westward                  1|Woody Allen
    --        111|Walt Disney               2|Warren Beatty
    --        112|Warner Bros.              3|Walter Matthau
    --        ...                           ...

SELECT distributors.name
    FROM   distributors
    WHERE  distributors.name LIKE 'W%'
UNION
SELECT actors.name
    FROM   actors
    WHERE  actors.name LIKE 'W%'

name
--------------
Walt Disney
Walter Matthau
Warner Bros.
Warren Beatty
Westward
Woody Allen
  </programlisting>
  
 </refsect1>
 
 <refsect1 id="R1-SQL-SELECT-3">
  <title>
   Compatibility
  </title>
  <para>
  </para>
  
  <refsect2 id="R2-SQL-SELECT-4">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    <acronym>Extensions</acronym>
   </title>

   <para>
<productname>Postgres</productname> allows one to omit 
the <command>FROM</command> clause from a query. This feature
was retained from the original PostQuel query language:
  <programlisting>
SELECT distributors.* WHERE name = 'Westwood';

    did|name
    ---+----------------
    108|Westward
  </programlisting>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-SELECT-5">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    <acronym>SQL92</acronym>
   </title>
   <para>
   </para>
   
   <refsect3 id="R3-SQL-SELECT-1">
    <refsect3info>
     <date>1998-04-15</date>
    </refsect3info>
    <title>
     SELECT Clause
    </title>
    <para>
     In the <acronym>SQL92</acronym> standard, the optional keyword "AS"
     is just noise and can be 
     omitted without affecting the meaning.
     The <productname>Postgres</productname> parser requires this keyword when
     renaming columns because the type extensibility features lead to
     parsing ambiguities
     in this context.</para>
     
    <para>
     In the <acronym>SQL92</acronym> standard, the new column name 
     specified in an
     "AS" clause may be referenced in GROUP BY and HAVING clauses.
     This is not currently
     allowed in <productname>Postgres</productname>.
    </para>
     
    <para>
     The DISTINCT ON phrase is not part of <acronym>SQL92</acronym>.
    </para>
   </refsect3>

   <refsect3 id="R3-SQL-UNION-1">
    <refsect3info>
     <date>1998-09-24</date>
    </refsect3info>
    <title>
     UNION Clause
    </title>
    <para>
     The <acronym>SQL92</acronym> syntax for UNION allows an
     additional CORRESPONDING BY clause:
     <synopsis> 
<replaceable class="PARAMETER">table_query</replaceable> UNION [ALL]
    [CORRESPONDING [BY (<replaceable class="PARAMETER">column</replaceable> [,...])]]
    <replaceable class="PARAMETER">table_query</replaceable>
     </synopsis></para>

    <para>
     The CORRESPONDING BY clause is not supported by
     <productname>Postgres</productname>.
    </para>
   </refsect3>
   
  </refsect2>
 </refsect1>
</refentry>

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