Re: unique primary key shared between multiple tables.

I just could not stand those names. Try this bit of SQL, which I am not recommending in practice:

CREATE TABLE RentalProperties
(property_nbr INTEGER NOT NULL
 private_owner_nbr INTEGER

     REFERENCES PrivateOwners(owner_nbr),  business_owner_nbr INTEGER

     REFERENCES BusinessOwners(owner_nbr),

• at least one owner CHECK (COALESCE (private_owner_nbr, business_owner_nbr) IS NOT NULL),
• at most one owner CHECK (COALESCE (private_owner_nbr, business_owner_nbr)
  • COALESCE (business_owner_nbr, private_owner_nbr)), ... );