How do you deal with polymorphism in a database?

Question

Example

I have Person, SpecialPerson, and User. Person and SpecialPerson are just people - they don\

Answer 1

If the User, Person and Special person all have the same foreign keys, then I would have a single table. Add a column called Type which is constrained to be User, Person or Special Person. Then based on the value of Type have constraints on the other optional columns.

For the object code it doesn't make much difference if you have the separate tables or multiple tables to represent polymorphism. However if you have to do SQL against the database, its much easier if the polymorphism is captured in single table...provided the foreign keys for the sub types are the same.