> which you probably _shouldn't_ be grouping on anyway.

This is frequently unavoidable, though. Or more precisely: it could be avoided with a sane database design, but the databases on which I have to work for my day job are the precise opposite of "well-designed", so grouping on complex expressions is unfortunately an inevitability.

I think it's for other reasons (and grouping on expressions is quite reasonable anyway).

It's (IIRC!) something to do with the situation of

  select x + y as x 
  from ...
  group by x

which x are we talking about? (Logically that example is crap because only the alias x makes sense, but something like that anyway).