Recently, a friend of mine asked me this question about the cardinality between a Department and Employee entities, I replied many to many, then he, my friend, showed me drawings from the internet which says otherwise, one to one. I objected the idea and told him don’t trust everything on the internet. However, after I examined books on Database Concepts I found the one to one relationship that my friend showed to me. As an example I took the following snapshot from “The Morgan Kaufmann Series in Database Management System: Database Modeling and Design _4th Edition_”
Fortunately, these guys mapped the entities depicting the foreign key relationship. Suppose after a year another employee is assigned to manage a department previously being managed by another employee. At this point are we going to update the mgr_id column and loose the history of the department and employee relationship?