The statement "A field cannot be aggregated and used for grouping at the same time." located in "17.5. Grouping and Aggregation Operations"
(see http://docbuilder.usersys.redhat.com/23162/#Grouping_and_Aggregate_Operations) is no longer true in JDG 7 and should be removed (this was a corner case that was not implemented in JDG 6.6 query engine and was rejected with an error during query parsing).

The note "A projection query in which all selected fields are aggregated and ..." should be expanded to mention another 2 special cases, besides the global aggregation case.

#2 "A grouping field can be used in an aggregation. This is a degenerated case in which the aggregation will be computed over a single data point, the value belonging to current group."

#3 "A query that selects only grouping fields but no aggregation fields is legal."