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From "Dag H. Wanvik (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-4909) Allow SELECT list column references not mentioned in GROUP BY if CR is functionally dependent
Date Sat, 20 Nov 2010 02:11:13 GMT
Allow SELECT list column references not mentioned in GROUP BY if CR is functionally dependent
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                 Key: DERBY-4909
                 URL: https://issues.apache.org/jira/browse/DERBY-4909
             Project: Derby
          Issue Type: Improvement
            Reporter: Dag H. Wanvik


Presently, Derby rejects column references in a SELECT .. GROUP BY if the column reference
isn't a grouping column.  This was correct SQL as per SQL 1992, but later versions of the
standard has loosened this to the present wording (e.g. SQL 2003, section 7.12 <query specification>,
SR 15:

 "If T is a grouped table, then let G be the set of grouping columns of T. In each <value
expression> contained in <select list> , each column reference that references a
column of T shall reference some column C that is functionally dependent on G or shall be
contained in an aggregated argument of a <set function specification> whose aggregation
query is QS."

This can be useful in certain queries, cf. the example below culled from http://rpbouman.blogspot.com/2007/05/debunking-group-by-myths.html

SELECT    f.film_id,
          f.title,
          COUNT(fa.actor_id)
FROM      film        f
LEFT JOIN film_actor  fa
ON        f.film_id = fa.film_id
GROUP BY  f.film_id;

In this case, f.title is functionally dependent on f.film_id (primary key), so the query is
correct according to SQL 2003 and later.

Derby requires that f.title also be specified as a grouping column, which is more verbose,
but can also have performance implications (although I didn't attempt to measure how this
could impact Derby yet), at least if the functional dependency analysis is not performed to
eliminate the extra grouping column. Do we do any such analysis?

Another example, a generated column would also be functionally dependent on the columns used
to compute it.


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