I don't think you can, dude. You're trying to say that propertyForRent.OwnerNo must exist in
either PrivateOwner.OwnerNo or BusinessOwner.OwnerNo and I don't think you can do that.
AFAIK (and I should be ashamed if I don't know this, nearly 6 years as an Oracle developer and all) a foreign key constraint can reference only one primary key.
If you want this, I humbly suggest that your data model is wrong. Perhaps you should merge your BusinessOwner and PrivateOwner into a single table, with OwnerNo as its primary key, and an extra column to identify whether the owner is private or commercial. You could then use two views to simulate the BusinessOwner and PrivateOwner tables if you wished:
Code:
CREATE VIEW BusinessOwner AS SELECT * FROM Owner WHERE Type='BUSINESS';
CREATE VIEW PrivateOwner AS SELECT * FROM Owner WHERE Type='PRIVATE';
That might help you out.