Restricting one type of record to just one user
So I am making a staff and devices tables, where staff members will be able to hold many types of devices (computers, tablets, etc.), but staff can only have one device type "Phone". I want to include phones from within the Devices table, but be able to prevent two phones from being assigned to one person.
P.S. Excuse if my title wasn't clear. It was kind of hard to describe in a title exactly what I want.