1. Entity / Relationship Diagrams Objects entities Classes entity sets Attributes are like in ODL. Relationships: like in ODL except - not associated with classes (I.e., first class citizens) - not necessarily binary Product address buys

2. address namessn Person buys makes employs Company Product namecategory stockprice name price

3. Multi-way Relationships Purchase How do we model a purchase relationship between buyers, products and stores? Product Person Store

4. Roles in Relationships Purchase What if we need an entity set twice in one relationship? Product Person Store salesperson buyer

5. Roles in Relationships Purchase Product Person Store salesperson buyer Note the multiplicity of the relationships: we cannot express all possibilities

6. Attributes on Relationships Purchase Product Person Store date

7. Design Principles Purchase Product Person What’s wrong? President PersonCountry Moral: be faithful!

8. What’s Wrong? Purchase Product Store date person personAddr Moral: don’t talk too much.

9. What’s Wrong? Purchase Product Person Store date Dates Moral: don’t complicate life more than it already is.

10. Do we really need 3-way relationships? Purchase Person Store Product StoreOf ProductOf BuyerOf Moral: Find a nice way to say things.