The EJBQL compiler needs to check whether the SELECT clause of a GROUP BY query
matches the requirement as specified in section "4.7 GROUP BY, HAVING" of the spec:
The requirements for the SELECT clause when GROUP BY is used follow those of
SQL: namely, any item that appears in the SELECT clause (other than as an
argument to an aggregate function) must also appear in the GROUP BY clause. In
forming the groups, null values are treated as the same for grouping purposes.