Better code structure

[aersam]

• SQL Execution is split between dataframe and endpoint, sometimes diff is a bit weird
• Do we really need groupby/joins/sortby? If yes: test and document

[dominikpeter]

We need sortby. Get the top n sorted by is a common use case. We need it for the recommender for sure.
Groupby and joins could be handled through the SQL Endpoint. Probably better to do there.

[aersam]

ok, I'll remove groupby and join then. and might do some test for sortby