Oh boy, you're going to have fun. You're about to learn all about SQL joins. Look them up in the MS SQL manual (w3schools provides a reasonable primer).
In short, there's no unique answer to your question. It depends on which type of join is appropriate to your tables and the particular query you are trying to compose, which itself depends on the way in which your data is normaisled into tables and how the tables are keyed.
Oh I read about that, but it seems that I was doing it wrong my code doesn't work. Maybe I should have posted it. Well i'll post it tomorrow hopefull too sleepy to do it now =.= by the way, you're right it was fun!!!
If you're in one of those predicaments where cls.__private attributes just aren't enough since they can easily be accessed through inst._cls__private, and you need something a little more secure, here's ...