I was following along (without peeking ahead) and I briefly thought "What about NULLs and empty joins?" But I figured, it is an idealized test. For example, what happens when a boss has a NULL department id? Would it be safe to say that they are in a different department than their underling? SQL says no.

Besides that, I think this is a great test. Personally, I start off a bit slower so I don't embarrass people that don't know SQL.