What is the foreign key between two children in the same hierarchy? I'm trying to map an entity relationship design into a relational database. Under STAFF entity, I have 2 children are BUSINESS MANAGER and EMPLOYEE.

My teacher requests to add a 1-M relationship SUPERVISE between Business_MGR and EMPLOYEE (a BUSINESS-MGR supervises 0, 1, or more employees, an EMPLOYEE is always supervised by one BUSINESS-MGR). 

What is the foreign key for the EMPLOYEE entity?
The id of a manager that you store in the supervisor column of employee.