Personally I always set the natural key with a not null and unique constraint, but create an artificial key for it as well. As an example, if we had a product table, the product_sku is defined as not null with a unique constraint on it, while product_id is the primary key which all other tables reference as a foreign key.
In the case of a many to many situation, I prefer to use a two column composite key. In the case of a many to many, i've never run into a case where I needed to reference a single row in that table without knowing about both sides of that relation.
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?