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?

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