select.sgml

关系型数据库 Postgresql 6.5.2

<refentry id="SQL-SELECT">
 <refmeta>
  <refentrytitle>
SELECT
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
SELECT
  </refname>
  <refpurpose>
   Retrieve rows from a table or view.
  </refpurpose></refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1998-09-24</date>
  </refsynopsisdivinfo>
  <synopsis>
SELECT [ALL|DISTINCT [ON <replaceable class="PARAMETER">column</replaceable>] ]
    <replaceable class="PARAMETER">expression</replaceable> [ AS
   <replaceable class="PARAMETER">name</replaceable> ] [, ...]
    [ INTO [TEMP] [TABLE] <replaceable class="PARAMETER">new_table</replaceable> ]
    [ FROM <replaceable class="PARAMETER">table</replaceable>
   [<replaceable class="PARAMETER">alias</replaceable> ] [, ...] ]
    [ WHERE <replaceable class="PARAMETER">condition</replaceable> ]
    [ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...] ]
    [ HAVING <replaceable class="PARAMETER">condition</replaceable> [, ...] ]
    [ { UNION [ALL] | INTERSECT | EXCEPT } <replaceable
    class="PARAMETER">select</replaceable> ]
    [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
    [ FOR UPDATE [OF class_name...]]
    [ LIMIT count [OFFSET|, count]]
  </synopsis>
  
  <refsect2 id="R2-SQL-SELECT-1">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   
   <para>
    <variablelist>
     <varlistentry>
      <term>
       <replaceable class="PARAMETER">expression</replaceable>
      </term>
      <listitem>
       <para>
	The name of a table's column or an expression.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term>
       <replaceable class="PARAMETER">name</replaceable>
      </term>
      <listitem>
       <para>
	Specifies another name for a column or an expression using
	the AS clause. <replaceable class="PARAMETER">name</replaceable>
	cannot be used in the WHERE
	condition. It can, however, be referenced in associated
	ORDER BY or GROUP BY clauses.
       </para>
      </listitem>
     </varlistentry>
     
    <varlistentry>
     <term>
     TEMP
     </term>
     <listitem>
      <para>
	The table is created unique to this session, and is
	automatically dropped on session exit.
      </para>
     </listitem>
    </varlistentry>

     <varlistentry>
      <term>
       <replaceable class="PARAMETER">new_table</replaceable>
      </term>
      <listitem>
       <para>
	If the INTO TABLE clause is specified, the result of the
	query will be stored in another table with the indicated
	name.
	The target table (<replaceable class="PARAMETER">new_table</replaceable>) will
	be created automatically and should not exist before this command.
        Refer to <command>SELECT INTO</command> for more information.

	<note>
	 <para>
	  The <command>CREATE TABLE AS</command> statement will also
	  create a new  table from a select query.
	 </para>
	</note>
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term>
       <replaceable class="PARAMETER">table</replaceable>
      </term>
      <listitem>
       <para>
	The name of an existing table referenced by the FROM clause.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term>
       <replaceable class="PARAMETER">alias</replaceable>
      </term>
      <listitem>
       <para>
	An alternate name for the preceding
	<replaceable class="PARAMETER">table</replaceable>.
	It is used for brevity or to eliminate ambiguity for joins
	within a single table.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term>
       <replaceable class="PARAMETER">condition</replaceable>
      </term>
      <listitem>
       <para>
	A boolean expression giving a result of true or false.
	See the WHERE clause.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term>
       <replaceable class="PARAMETER">column</replaceable>
      </term>
      <listitem>
       <para>
	The name of a table's column.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term>
       <replaceable class="PARAMETER">select</replaceable>
      </term>
      <listitem>
       <para>
	A select statement with all features except the ORDER BY clause.
       </para>
      </listitem>
     </varlistentry>
     
    </variablelist>
   </para>
  </refsect2>
  
  <refsect2 id="R2-SQL-SELECT-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>
	
	<variablelist>
	 <varlistentry>
	  <term>
	   Rows
	  </term>
	  <listitem>
	   <para>
		The complete set of rows resulting from the query specification.
	   </para>
	  </listitem>
	 </varlistentry>
	 
	 <varlistentry>
	  <term>
	   <returnvalue><replaceable>count</replaceable></returnvalue>
	  </term>
	  <listitem>
	   <para>
		The count of rows returned by the query.
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>
 
 <refsect1 id="R1-SQL-SELECT-1">
  <refsect1info>
   <date>1998-09-24</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>SELECT</command> will return rows from one or more tables.
   Candidates for selection are rows which satisfy the WHERE condition;
   if WHERE is omitted, all rows are candidates.</para>
  <para>
   <command>DISTINCT</command> will eliminate all duplicate rows from the
   selection.
   <command>DISTINCT ON <replaceable class="PARAMETER">column</replaceable></command> will eliminate all duplicates in the specified column; this is
equivalent to using <command>GROUP BY <replaceable
     class="PARAMETER">column</replaceable></command>.  <command>ALL</command> will return all candidate rows,
including duplicates.</para>

  <para>
   The GROUP BY clause allows a user to divide a table
   conceptually into groups. (See GROUP BY clause).</para>
   
  <para>
   The HAVING clause specifies a grouped table derived by the
   elimination of groups from the result of the previously
   specified clause. (See HAVING clause).</para>
   
  <para>
   The ORDER BY clause allows a user to specify that he/she
   wishes the rows sorted according to the ASCending or 
   DESCending mode operator. (See 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).</para>
   
  <para>
   The INTERSECT give you the rows that are common to both queries.
   (See INTERSECT clause).</para>
   
  <para>
   The EXCEPT give you the rows in the upper query not in the lower query.
   (See EXCEPT clause).</para>
   
  <para>
   The FOR UPDATE clause allows the SELECT statement to perform 
   exclusive locking of selected rows.
   (See EXCEPT clause).</para>
   
  <para>
   The LIMIT...OFFSET clause allows control over which rows are
   returned by the query.</para>   

  <para>
   You must have SELECT privilege to a table to read its values
   (See <command>GRANT</command>/<command>REVOKE</command> statements).
</para>
   
  <refsect2 id="R2-SQL-WHERE-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    WHERE Clause
   </title>
   <para>
    The optional WHERE condition has the general form:
    
    <synopsis>
WHERE <replaceable class="PARAMETER">expr</replaceable> <replaceable
class="PARAMETER">ETER">c</replaceable>e<replaceable class="PARAMETER">"PAR</replaceable>replaceable> [ <replaceable
class="PARAMETER">log_op</replaceable> ... ]
    </synopsis>
    
    where <replaceable class="PARAMETER">cond_op</replaceable> can be
    one of: =, &lt;, &lt;=, &gt;, &gt;= or &lt;&gt;,
    a conditional operator like ALL, ANY, IN, LIKE, et cetera or a
    locally-defined operator, 
    and <replaceable class="PARAMETER">log_op</replaceable> can be one 
    of: AND, OR, NOT.
    The comparison returns either TRUE or FALSE and all
    instances will be discarded
    if the expression evaluates to FALSE.
   </para>
  </refsect2>
  
  <refsect2 id="R2-SQL-GROUPBY-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    GROUP BY Clause
   </title>
   <para>
    GROUP BY specifies a grouped table derived by the application
    of this clause:
    <synopsis>
GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...]
    </synopsis></para>
    <para>
     GROUP BY will condense into a single row all rows that share the same values for the
     grouped columns; aggregates return values derived from all rows that make up the group.  The value returned for an ungrouped
     and unaggregated column is dependent on the order in which rows happen to be read from the database.
    </para>
</refsect2>
    
  <refsect2 id="R2-SQL-HAVING-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    HAVING Clause
   </title>
   <para>
    The optional HAVING condition has the general form:
    
    <synopsis>
HAVING <replaceable class="PARAMETER">cond_expr</replaceable>
    </synopsis>
    
    where <replaceable class="PARAMETER">cond_expr</replaceable> is the same
    as specified for the WHERE clause.</para>
    
   <para>
    HAVING specifies a grouped table derived by the elimination
    of groups from the result of the previously specified clause
    that do not meet the <replaceable class="PARAMETER">cond_expr</replaceable>.</para>
    
   <para>
    Each column referenced in 
<replaceable class="PARAMETER">cond_expr</replaceable> shall unambiguously
    reference a grouping column.
   </para>
  </refsect2>
  
  <refsect2 id="R2-SQL-ORDERBYCLAUSE-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    ORDER BY Clause
   </title>
   <para>
    <synopsis>
ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...]
    </synopsis></para>
    
   <para>
    <replaceable class="PARAMETER">column</replaceable> can be either a column
    name or an ordinal number.</para>
   <para>
    The ordinal numbers refers to the ordinal (left-to-right) position
    of the column. This feature makes it possible to define an ordering
    on the basis of a column that does not have a proper name.
    This is never absolutely necessary because it is always possible