Apologies for the self-plug, but you might find some stuff in the later-stage Aggregates and Recursive exercises on http://pgexercises.com http://pgexercises.co

Apologies for the self-plug, but you might find some stuff in the later-stage Aggregates and Recursive exercises on http://pgexercises.com http://pgexercises.com . There's a bunch of stuff on CTEs and window functions in there.