diff options
Diffstat (limited to 'doc/src/sgml/ref/select.sgml')
| -rw-r--r-- | doc/src/sgml/ref/select.sgml | 21 |
1 files changed, 12 insertions, 9 deletions
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml index f2afa38b7c9..070f8b43d0f 100644 --- a/doc/src/sgml/ref/select.sgml +++ b/doc/src/sgml/ref/select.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.22 1999/08/06 13:50:31 thomas Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.23 1999/12/13 17:39:38 tgl Exp $ Postgres documentation --> @@ -202,10 +202,10 @@ SELECT [ ALL | DISTINCT [ ON <replaceable class="PARAMETER">column</replaceable> <para> <command>DISTINCT</command> will eliminate all duplicate rows from the - selection. + result. <command>DISTINCT ON <replaceable class="PARAMETER">column</replaceable></command> will eliminate all duplicates in the specified column; this is - equivalent to using + similar to using <command>GROUP BY <replaceable class="PARAMETER">column</replaceable></command>. <command>ALL</command> will return all candidate rows, including duplicates. @@ -320,11 +320,13 @@ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...] <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. + 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 + ungrouped columns except within aggregate functions, since there + would be more than one possible value to return for an ungrouped column. </para> </refsect2> @@ -354,7 +356,8 @@ HAVING <replaceable class="PARAMETER">cond_expr</replaceable> <para> Each column referenced in <replaceable class="PARAMETER">cond_expr</replaceable> shall unambiguously - reference a grouping column. + reference a grouping column, unless the reference appears within an + aggregate function. </para> </refsect2> |