There is a table
users with autoincrement primary key
There is also a table
employees (every employee is a user but not vice versa). The
employees primary key is
userid. The field
userid is not autoincrement.
employees.userid is a foreign key for
Thus every employee's primary key value is also
users primary key value.
Now consider a third table rows of which should refer to an employee.
The question: Should the third table refer to
users.id or to