Binary vs. Ternary Relationships (Contd.)
Previous example illustrated a case when 2 binary relationships were better than a ternary relationship.
An example in the other direction: a ternary relation Contracts relates entity sets Parts, Departments and Suppliers, and has descriptive attribute qty. No combination of binary relationships is an adequate substitute:
- S “can-supply’’ P, D “needs’’ P, and D “deals-with’’ S does not imply that D has agreed to buy P from S.
- How do we record qty?