diff options
Diffstat (limited to 'doc/src/sgml/queries.sgml')
| -rw-r--r-- | doc/src/sgml/queries.sgml | 20 |
1 files changed, 10 insertions, 10 deletions
diff --git a/doc/src/sgml/queries.sgml b/doc/src/sgml/queries.sgml index 12f043b95f..b4ec30773d 100644 --- a/doc/src/sgml/queries.sgml +++ b/doc/src/sgml/queries.sgml @@ -1,4 +1,4 @@ -<!-- $Header: /cvsroot/pgsql/doc/src/sgml/queries.sgml,v 1.18 2002/10/20 05:05:46 tgl Exp $ --> +<!-- $Header: /cvsroot/pgsql/doc/src/sgml/queries.sgml,v 1.19 2002/11/11 20:14:03 petere Exp $ --> <chapter id="queries"> <title>Queries</title> @@ -668,7 +668,7 @@ SELECT <replaceable>select_list</replaceable> order in which the columns are listed does not matter. The purpose is to reduce each group of rows sharing common values into one group row that is representative of all rows in the group. - This is done to eliminate redundancy in the output and/or obtain + This is done to eliminate redundancy in the output and/or compute aggregates that apply to these groups. For instance: <screen> <prompt>=></> <userinput>SELECT * FROM test1;</> @@ -694,7 +694,12 @@ SELECT <replaceable>select_list</replaceable> In the second query, we could not have written <literal>SELECT * FROM test1 GROUP BY x</literal>, because there is no single value for the column <literal>y</> that could be associated with each - group. In general, if a table is grouped, columns that are not + group. The grouped-by columns can be referenced in the select list since + they have a known constant value per group. + </para> + + <para> + In general, if a table is grouped, columns that are not used in the grouping cannot be referenced except in aggregate expressions. An example with aggregate expressions is: <screen> @@ -712,11 +717,6 @@ SELECT <replaceable>select_list</replaceable> linkend="functions-aggregate">. </para> - <para> - The grouped-by columns can be referenced in the select list since - they have a known constant value per group. - </para> - <tip> <para> Grouping without aggregate expressions effectively calculates the @@ -740,7 +740,7 @@ SELECT product_id, p.name, (sum(s.units) * p.price) AS sales in the <literal>GROUP BY</> clause since they are referenced in the query select list. (Depending on how exactly the products table is set up, name and price may be fully dependent on the - product ID, so the additional groups could theoretically be + product ID, so the additional groupings could theoretically be unnecessary, but this is not implemented yet.) The column <literal>s.units</> does not have to be in the <literal>GROUP BY</> list since it is only used in an aggregate expression @@ -828,7 +828,7 @@ SELECT product_id, p.name, (sum(s.units) * (p.price - p.cost)) AS profit </para> <sect2 id="queries-select-list-items"> - <title>Select List Items</title> + <title>Select-List Items</title> <para> The simplest kind of select list is <literal>*</literal> which |