How do you deal with polymorphism in a database?

Example

I have Person, SpecialPerson, and User. Person and SpecialPerson are just people - they don't have a user name or password on a site, but they are stored in a database for record keeping. User has all of the same data as Person and potentially SpecialPerson, along with a user name and password as they are registered with the site.

How would you address this problem? Would you have a Person table which stores all data common to a person and use a key to look up their data in SpecialPerson (if they are a special person) and User (if they are a user) and vice-versa?