Consider: SELECT c.registration_no, c.car_make, p.part_no FROM car c JOIN parts p ON ( p.registration_no = c.registration_no) WHERE registration_no = <some_var>;
versus: SELECT c.registration_no, c.car_make, p.part_no FROM car c JOIN parts p ON ( p.id = c.id) WHERE registration_no = <some_var>;
Why join on id when registration_no is better?
I believe you are mistaken if you think there are absolute rules you can cling to here. But even then I would lean toward calling primary keys an internal implementation detail that should be under the full control of the database in which they are directly used. Artifical "natural" keys I would lean toward turning into, possibly unique, attributes. Inherent "natural"
keys get some consideration for using directly.
The issue arise more, say, in a many-to-many situation. Do you define the PK of the linking table as a two-column composite key or do you introduce a third, serial, field to stand in for the pair?