Entity / Relationship Diagrams

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. name category name price makes Company Product stockprice buys employs Person name ssn address

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

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

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

6. Attributes on Relationships date Product Purchase Store Person

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

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

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

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