Access Database Design and Programming, Second Edition

Relationships Between Entities

If we are going to model a database as a collection of entity sets (tables), then we need to also 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).

Figure 2-1. The LIBRARY entity-relationship diagram

Note that each entity class is denoted by a rectangle, and each attribute by an ellipse. The relations are denoted by diamonds. We have included the Contributors entity class in this model merely to illustrate a special type of relationship. In particular, since a contributor is considered an author, there is an IsA relationship between the two entity classes.

The model represented by an E/R diagram is sometimes referred to as a semantic model, since it describes much of the meaning of the database.

Types of Relationships ...