Relational Algebra - Cartesian Product vs Natural Join?

Question

I am studying for exams and am failing to find a solid criteria by which I can determine if the Cartesian Product x is to be used or if Natural Join |X|

Answer 1

I think there are two extreme situation:

1. there are no duplicate rows in inner join: inner join is equal to intersect (i mean only the result). distinct inner join ~ intersect

2. there are no common feature in the inner join: inner join is equal to Cartesian product.