As someone who uses a LOT of SQL, and who has taught people before, I like this as a start, but it definitely needs more visualzation, and I recommend how sqlzoo represents tables in the top right with an overlay, its a simple and clear break down.

Something like http://blog.codinghorror.com/a-visual-explanation-of-sql-joi... may help with additional explanations.

I also think that things like sargability need to be discussed early on because while sql/set based stuff can be odd to grok at first, knowing why you are doing the things you do is important in laying the right foundations.

This is a self-plug, but you might find http://pgexercises.com useful as well.

Returns a "webpage not available" for me. However, if I access it from http://sqlzoo.net/wiki/Main_Page it seems fine.