Supposing you have a table Vehicle, you might need tables Truck and Motorcycle to hold information specific to only some of the rows in Vehicle.

You're describing here a design rule able to emulate an heritage, which is mostly a matter of methodology/dba experience.

With a (real) OODBMS, this kind of trickery relational modelisation is not required.

____
HTH, Dominique
My two favorites:
If the only tool you have is a hammer, you will see every problem as a nail. --Abraham Maslow
Bien faire, et le faire savoir...

In reply to Re^3: OO concepts and relational databases by dfaure
in thread OO concepts and relational databases by dragonchild

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