Login system: how is it possible to model multiple user types identified by only one account?
I don't think you should make tables to represent Roles (User, Manager, Seller, etc) I think you could implement something more scalable like this:



Completely forget about the "user-centric" model and think to a lower level, Individuals. Individuals can have any number of Accounts and Accounts can have any number of Roles. Individuals can be represented on any number of abstractions around the database or even across databases.

What do you think?