Hi,

I have the following constraints
An employee can manage @ most 1 department
A department can have @ most 1 manager

Thus by the rules of "pick a relationship and include the fk in x etc.", I have

Employee_ID(PK) | Department_Id

Data =
1 - DA
2 - DB
3 - DB
4 - DA

Clearly the above violations have been violated. I do not understand

You would need to add a UNIQUE constraint on employee.department_id to prevent setting up many employees as manager of same department.