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2. The Entity-Relationship Model of a Da... > 2.4. Relationships Between Entities

Relationships Between Entities

If we are going to model a database as a collection of entity sets (tables), then we also need to describe the relationships between these entity sets. For instance, an author relationship exists between a book and the authors who wrote that book. We might call this relationship WrittenBy. Thus, Hamlet is WrittenBy Shakespeare.

It is possible to draw a diagram, called an entity-relationship diagram, or E/R diagram,to illustrate the entity classes in a database model, along with their attributes and relationships. Figure 2-1 shows the LIBRARY E/R diagram, with an additional entity class called Contributors (a contributor may be someone who contributes to or writes only a very small portion of a book, and thus may not be accorded all of the rights of an author, such as a royalty).


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