[Added: Prolog SQL has some good info. You should definately avoid re-inventing a wheel. The real Prolog people have already been hacking on this problem.]

I was thinking about this yesterday. It seemed most natural that you'd patch AI::Prolog to be able to query this stuff. You might want to introduce a new built-in function to represent joins. You've got a problem in that SQL relations have no order to columns so you'll either need to ignore that or instruct AI::Prolog in what order to read columns when querying the database.

% A "normal" lookup
city('New York', Cityid), usr(Cityid, Name, Id).

% An id-less query. Maybe the engine notices the join and transforms t
+his into the prior behind the scenes.
% Maybe an invented built-in like references/4 cause the transformatio
+n.
:- references( 'USR', 'CITY_ID', 'USR', 'CITY_ID' ).
usr( city( 'New York' ), Name ).
[download]