diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/ref/select.sgml | 29 |
1 files changed, 21 insertions, 8 deletions
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml index 070f8b43d0f..ed180ac91c1 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 1.23 1999/12/13 17:39:38 tgl Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.24 2000/01/27 18:11:25 tgl Exp $ Postgres documentation --> @@ -22,7 +22,7 @@ Postgres documentation <date>1999-07-20</date> </refsynopsisdivinfo> <synopsis> -SELECT [ ALL | DISTINCT [ ON <replaceable class="PARAMETER">column</replaceable> ] ] +SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) ] ] <replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">name</replaceable> ] [, ...] [ INTO [ TEMPORARY | TEMP ] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ] [ FROM <replaceable class="PARAMETER">table</replaceable> [ <replaceable class="PARAMETER">alias</replaceable> ] [, ...] ] @@ -201,17 +201,30 @@ SELECT [ ALL | DISTINCT [ ON <replaceable class="PARAMETER">column</replaceable> </para> <para> - <command>DISTINCT</command> will eliminate all duplicate rows from the + <command>DISTINCT</command> will eliminate duplicate rows from the result. - <command>DISTINCT ON <replaceable class="PARAMETER">column</replaceable></command> - will eliminate all duplicates in the specified column; this is - similar to using - <command>GROUP BY <replaceable class="PARAMETER">column</replaceable></command>. - <command>ALL</command> will return all candidate rows, + <command>ALL</command> (the default) will return all candidate rows, including duplicates. </para> <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 + unless <command>ORDER BY</command> is used to ensure that the desired + row appears first. For example, + <programlisting> + SELECT DISTINCT ON (location) location, time, report + FROM weatherReports + ORDER BY location, time DESC; + </programlisting> + retrieves the most recent weather report for each location. But if + we had not used ORDER BY to force descending order of time values + for each location, we'd have gotten a report of unpredictable age + for each location. + </para> + + <para> The GROUP BY clause allows a user to divide a table conceptually into groups. (See <xref linkend="sql-groupby" endterm="sql-groupby-title">.) |