At what normalisation stage is it recommend to split say Customers and Employees into a People table and then link to the unique attributes of Customers and Employees? I can't seem to find the Functional Dependency to justify the change. This makes sense in OOP where you use inheritance but I don’t understand why it is done in database schemas.
This is an example of what I mean, possibly with more unique attributes:
Customers: Cid, FName, LName, AddId, CustType Employees: Eid, FName, LName, AddId, EmpType
People : FName, LName, AddId Customers: PeopleId, CustType Employers: PeopleId, EmpType
The current answers explain why it is recommended to move data to a People table; however, at what normalisation stage does it become a FD to justify creating the extra table?