A school employs a number of staff. A school runs a number of courses. During the course of a season a football team plays a number of matches. Some of these are home matches and some are away matches. Matches always involve two teams one playing away and one at home.
Teams belong to a variety of leagues each of which is subdivided into divisions.
A team may only belong to one league and one division of the league. A company maintains purchase order information. Customers place a number of orders. An order can only arise from a single customer. An order can be for a number of parts.