[AlisdairO]

(self plug) try pgexercises.com. It's in a very similar vein to windowfunctions - I understand it was one of the original inspirations - but it covers a much wider range of material, with a gradual growth from very basic stuff up to slightly advanced.

[mercer]

I'll vouch for this plug! I've spent quite a bit of time learning SQL (and Postgres specific stuff), and this was one of my favorite resources.

I can also recommend Learning SQL by Alan Beaulieu, which was recommended in some HN comment. After looking into various books, some of them Postgres specific, I found this one to be the best for learning from scratch.