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Diffstat (limited to 'doc/src/sgml/queries.sgml')
| -rw-r--r-- | doc/src/sgml/queries.sgml | 54 |
1 files changed, 54 insertions, 0 deletions
diff --git a/doc/src/sgml/queries.sgml b/doc/src/sgml/queries.sgml index bc0b3cc9fe1..834b83b5098 100644 --- a/doc/src/sgml/queries.sgml +++ b/doc/src/sgml/queries.sgml @@ -1372,6 +1372,55 @@ GROUP BY GROUPING SETS ( </programlisting> </para> + <para> + <indexterm zone="queries-grouping-sets"> + <primary>ALL</primary> + <secondary>GROUP BY ALL</secondary> + </indexterm> + <indexterm zone="queries-grouping-sets"> + <primary>DISTINCT</primary> + <secondary>GROUP BY DISTINCT</secondary> + </indexterm> + When specifying multiple grouping items together, the final set of grouping + sets might contain duplicates. For example: +<programlisting> +GROUP BY ROLLUP (a, b), ROLLUP (a, c) +</programlisting> + is equivalent to +<programlisting> +GROUP BY GROUPING SETS ( + (a, b, c), + (a, b), + (a, b), + (a, c), + (a), + (a), + (a, c), + (a), + () +) +</programlisting> + If these duplicates are undesirable, they can be removed using the + <literal>DISTINCT</literal> clause directly on the <literal>GROUP BY</literal>. + Therefore: +<programlisting> +GROUP BY <emphasis>DISTINCT</emphasis> ROLLUP (a, b), ROLLUP (a, c) +</programlisting> + is equivalent to +<programlisting> +GROUP BY GROUPING SETS ( + (a, b, c), + (a, b), + (a, c), + (a), + () +) +</programlisting> + This is not the same as using <literal>SELECT DISTINCT</literal> because the output + rows may still contain duplicates. If any of the ungrouped columns contains NULL, + it will be indistinguishable from the NULL used when that same column is grouped. + </para> + <note> <para> The construct <literal>(a, b)</literal> is normally recognized in expressions as @@ -1561,7 +1610,12 @@ SELECT a "from", b + c AS sum FROM ... <title><literal>DISTINCT</literal></title> <indexterm zone="queries-distinct"> + <primary>ALL</primary> + <secondary>SELECT ALL</secondary> + </indexterm> + <indexterm zone="queries-distinct"> <primary>DISTINCT</primary> + <secondary>SELECT DISTINCT</secondary> </indexterm> <indexterm zone="queries-distinct"> |