<!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.25 2000/02/21 01:13:52 tgl Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.26 2000/03/15 23:31:19 tgl Exp $
Postgres documentation
-->
<refsect2 id="R2-SQL-SELECT-1">
<refsect2info>
- <date>1998-09-24</date>
+ <date>2000-03-15</date>
</refsect2info>
<title>
Inputs
<listitem>
<para>
Specifies another name for a column or an expression using
- the AS clause. This name is primarily used to label the output
- column. The <replaceable class="PARAMETER">name</replaceable>
- cannot be used in the WHERE, GROUP BY, or HAVING clauses.
- It can, however, be referenced in ORDER BY clauses.
+ the AS clause. This name is primarily used to label the column
+ for display. It can also be used to refer to the column's value in
+ ORDER BY and GROUP BY clauses. But the
+ <replaceable class="PARAMETER">name</replaceable>
+ cannot be used in the WHERE or HAVING clauses; write out the
+ expression instead.
</para>
</listitem>
</varlistentry>
<term>TEMP</term>
<listitem>
<para>
- The table is created unique to this session, and is
+ If TEMPORARY or TEMP is specified,
+ the table is created unique to this session, and is
automatically dropped on session exit.
</para>
</listitem>
<listitem>
<para>
If the INTO TABLE clause is specified, the result of the
- query will be stored in another table with the indicated
+ query will be stored in a new 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.
+ be created automatically and must not exist before this command.
Refer to <command>SELECT INTO</command> for more information.
<note>
<term><replaceable class="PARAMETER">select</replaceable></term>
<listitem>
<para>
- A select statement with all features except the ORDER BY clause.
+ A select statement with all features except the ORDER BY and
+ LIMIT clauses.
</para>
</listitem>
</varlistentry>
<refsect1 id="R1-SQL-SELECT-1">
<refsect1info>
- <date>1998-09-24</date>
+ <date>2000-03-15</date>
</refsect1info>
<title>
Description
<para>
<command>DISTINCT ON</command> eliminates rows that match on all the
specified expressions, keeping only the first row of each set of
- duplicates. Note that "the first row" of each set is unpredictable
+ duplicates. The DISTINCT ON expressions are interpreted using the
+ same rules as for ORDER BY items; see below.
+ Note that "the first row" of each set is unpredictable
unless <command>ORDER BY</command> is used to ensure that the desired
row appears first. For example,
<programlisting>
<para>
The GROUP BY clause allows a user to divide a table
- conceptually into groups.
+ into groups of rows that match on one or more values.
(See <xref linkend="sql-groupby" endterm="sql-groupby-title">.)
</para>
<para>
- The HAVING clause specifies a grouped table derived by the
- elimination of groups from the result of the previously
- specified clause.
+ The HAVING clause allows selection of only those groups of rows
+ meeting the specified condition.
(See <xref linkend="sql-having" endterm="sql-having-title">.)
</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.
+ The ORDER BY clause causes the returned rows to be sorted in a specified
+ order. If ORDER BY is not given, the rows are returned in whatever order
+ the system finds cheapest to produce.
(See <xref linkend="sql-orderby-title" endterm="sql-orderby-title">.)
</para>
<refsect2 id="SQL-WHERE">
<refsect2info>
- <date>1998-09-24</date>
+ <date>2000-03-15</date>
</refsect2info>
<title id="sql-where-title">
WHERE Clause
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.
+ SELECT will ignore all rows for which the WHERE condition does not return
+ TRUE.
</para>
</refsect2>
<refsect2 id="SQL-GROUPBY">
<refsect2info>
- <date>1998-09-24</date>
+ <date>2000-03-15</date>
</refsect2info>
<title id="sql-groupby-title">
GROUP BY Clause
</para>
<para>
- GROUP BY will condense into a single row all rows that share the
+ GROUP BY will condense into a single row all selected rows that share the
same values for the grouped columns. Aggregate functions, if any,
are computed across all rows making up each group, producing a
separate value for each group (whereas without GROUP BY, an
aggregate produces a single value computed across all the selected
- rows). When GROUP BY is present, it is not valid to refer to
+ rows). When GROUP BY is present, it is not valid for the SELECT
+ output expression(s) to refer to
ungrouped columns except within aggregate functions, since there
would be more than one possible value to return for an ungrouped column.
</para>
+
+ <para>
+ An item in GROUP BY can also be the name or ordinal number of an output
+ column (SELECT expression), or it can be an arbitrary expression formed
+ from input-column values. In case of ambiguity, a GROUP BY name will
+ be interpreted as an input-column name rather than an output column name.
+ </para>
</refsect2>
<refsect2 id="SQL-HAVING">
<refsect2info>
- <date>1998-09-24</date>
+ <date>2000-03-15</date>
</refsect2info>
<title id="sql-having-title">
HAVING Clause
<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>
+ of group rows that do not satisfy the
+ <replaceable class="PARAMETER">cond_expr</replaceable>.
+ HAVING is different from WHERE:
+ WHERE filters individual rows before application of GROUP BY,
+ while HAVING filters group rows created by GROUP BY.
+ </para>
<para>
Each column referenced in
<refsect2 id="SQL-ORDERBY">
<refsect2info>
- <date>1998-09-24</date>
+ <date>2000-03-15</date>
</refsect2info>
<title id="sql-orderby-title">
ORDER BY Clause
</synopsis></para>
<para>
- <replaceable class="PARAMETER">column</replaceable> can be either a column
- name or an ordinal number.
+ <replaceable class="PARAMETER">column</replaceable> can be either a
+ result 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
+ of the result 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
- to assign a name to a calculated column using the AS clause, e.g.:
+ to assign a name to a result column using the AS clause, e.g.:
<programlisting>
SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;
</programlisting></para>
<programlisting>
SELECT name FROM distributors ORDER BY code;
</programlisting>
+ Note that if an ORDER BY item is a simple name that matches both
+ a result column name and an input column name, ORDER BY will interpret
+ it as the result column name. This is the opposite of the choice that
+ GROUP BY will make in the same situation. This inconsistency is
+ mandated by the SQL92 standard.
</para>
<para>
where
<replaceable class="PARAMETER">table_query</replaceable>
- specifies any select expression without an ORDER BY clause.
+ specifies any select expression without an ORDER BY or LIMIT clause.
</para>
<para>
where
<replaceable class="PARAMETER">table_query</replaceable>
- specifies any select expression without an ORDER BY clause.
+ specifies any select expression without an ORDER BY or LIMIT clause.
</para>
<para>
where
<replaceable class="PARAMETER">table_query</replaceable>
- specifies any select expression without an ORDER BY clause.
+ specifies any select expression without an ORDER BY or LIMIT clause.
</para>
<para>
</para>
<para>
- As of Postgres 7.0, the
+ As of PostgreSQL 7.0, 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
The DISTINCT ON phrase is not part of <acronym>SQL92</acronym>.
Nor are LIMIT and OFFSET.
</para>
+
+ <para>
+ In <acronym>SQL92</acronym>, an ORDER BY clause may only use result
+ column names or numbers, while a GROUP BY clause may only use input
+ column names.
+ <productname>Postgres</productname> extends each of these clauses to
+ allow the other choice as well (but it uses the standard's interpretation
+ if there is ambiguity).
+ <productname>Postgres</productname> also allows both clauses to specify
+ arbitrary expressions. Note that names appearing in an expression will
+ always be taken as input-column names, not as result-column names.
+ </para>
</refsect3>
<refsect3 id="R3-SQL-UNION-1">
*
*
* IDENTIFICATION
- * $Header: /cvsroot/pgsql/src/backend/parser/parse_clause.c,v 1.56 2000/03/14 23:06:32 thomas Exp $
+ * $Header: /cvsroot/pgsql/src/backend/parser/parse_clause.c,v 1.57 2000/03/15 23:31:04 tgl Exp $
*
*-------------------------------------------------------------------------
*/
* list as a "resjunk" node.
*
* node the ORDER BY, GROUP BY, or DISTINCT ON expression to be matched
- * tlist the existing target list (NB: this cannot be NIL, which is a
- * good thing since we'd be unable to append to it...)
- * clause identifies clause type (mainly for error messages).
+ * tlist the existing target list (NB: this will never be NIL, which is a
+ * good thing since we'd be unable to append to it if it were...)
+ * clause identifies clause type being processed.
*/
static TargetEntry *
findTargetlistEntry(ParseState *pstate, Node *node, List *tlist, int clause)
/*----------
* Handle two special cases as mandated by the SQL92 spec:
*
- * 1. ORDER BY ColumnName
+ * 1. Bare ColumnName (no qualifier or subscripts)
* For a bare identifier, we search for a matching column name
* in the existing target list. Multiple matches are an error
* unless they refer to identical values; for example,
* but not SELECT a AS b, b FROM table ORDER BY b
* If no match is found, we fall through and treat the identifier
* as an expression.
- * We do NOT attempt this match for GROUP BY, since it is clearly
- * contrary to the spec to use an output column name in preference
- * to an underlying column name in GROUP BY. DISTINCT ON isn't in
- * the standard, so we can do what we like there; we choose to make
- * it work like GROUP BY.
+ * For GROUP BY, it is incorrect to match the grouping item against
+ * targetlist entries: according to SQL92, an identifier in GROUP BY
+ * is a reference to a column name exposed by FROM, not to a target
+ * list column. However, many implementations (including pre-7.0
+ * PostgreSQL) accept this anyway. So for GROUP BY, we look first
+ * to see if the identifier matches any FROM column name, and only
+ * try for a targetlist name if it doesn't. This ensures that we
+ * adhere to the spec in the case where the name could be both.
+ * DISTINCT ON isn't in the standard, so we can do what we like there;
+ * we choose to make it work like ORDER BY, on the rather flimsy
+ * grounds that ordinary DISTINCT works on targetlist entries.
*
- * 2. ORDER BY/GROUP BY/DISTINCT ON IntegerConstant
+ * 2. IntegerConstant
* This means to use the n'th item in the existing target list.
* Note that it would make no sense to order/group/distinct by an
* actual constant, so this does not create a conflict with our
* extension to order/group by an expression.
- * I believe that GROUP BY column-number is not sanctioned by SQL92,
- * but since the standard has no other behavior defined for this
- * syntax, we may as well continue to support our past behavior.
+ * GROUP BY column-number is not allowed by SQL92, but since
+ * the standard has no other behavior defined for this syntax,
+ * we may as well accept this common extension.
*
- * Note that pre-existing resjunk targets must not be used in either case.
+ * Note that pre-existing resjunk targets must not be used in either case,
+ * since the user didn't write them in his SELECT list.
+ *
+ * If neither special case applies, fall through to treat the item as
+ * an expression.
*----------
*/
- if (clause == ORDER_CLAUSE &&
- IsA(node, Ident) && ((Ident *) node)->indirection == NIL)
+ if (IsA(node, Ident) && ((Ident *) node)->indirection == NIL)
{
char *name = ((Ident *) node)->name;
- foreach(tl, tlist)
+
+ if (clause == GROUP_CLAUSE)
{
- TargetEntry *tle = (TargetEntry *) lfirst(tl);
- Resdom *resnode = tle->resdom;
+ /*
+ * In GROUP BY, we must prefer a match against a FROM-clause
+ * column to one against the targetlist. Look to see if there is
+ * a matching column. If so, fall through to let transformExpr()
+ * do the rest. NOTE: if name could refer ambiguously to more
+ * than one column name exposed by FROM, colnameRangeTableEntry
+ * will elog(ERROR). That's just what we want here.
+ */
+ if (colnameRangeTableEntry(pstate, name) != NULL)
+ name = NULL;
+ }
- if (!resnode->resjunk &&
- strcmp(resnode->resname, name) == 0)
+ if (name != NULL)
+ {
+ foreach(tl, tlist)
{
- if (target_result != NULL)
+ TargetEntry *tle = (TargetEntry *) lfirst(tl);
+ Resdom *resnode = tle->resdom;
+
+ if (!resnode->resjunk &&
+ strcmp(resnode->resname, name) == 0)
{
- if (! equal(target_result->expr, tle->expr))
- elog(ERROR, "%s '%s' is ambiguous",
- clauseText[clause], name);
+ if (target_result != NULL)
+ {
+ if (! equal(target_result->expr, tle->expr))
+ elog(ERROR, "%s '%s' is ambiguous",
+ clauseText[clause], name);
+ }
+ else
+ target_result = tle;
+ /* Stay in loop to check for ambiguity */
}
- else
- target_result = tle;
- /* Stay in loop to check for ambiguity */
}
+ if (target_result != NULL)
+ return target_result; /* return the first match */
}
- if (target_result != NULL)
- return target_result; /* return the first match */
}
if (IsA(node, A_Const))
{
projects / postgresql / commitdiff