Staff to head one section at a time using MySQL

I have 3 tables namely:

1. department_heads

  • DepartmentID
  • StaffID

2. sub_department_heads

  • SubDepartmentID
  • StaffID

3. branch_managers

  • BranchID
  • StaffID

Now what I want is not have staff being a department head as well as a branch manager or sub department head and vise verse.

Staff can only belong in only one table.

Kindly advice on how i can structure my tables. Thanks in advance.