Chapter 2. The Entity-Relationship Model of a Database

Let us begin our discussion of database design by looking at an informal database model called the entity-relationship model . This model of a relational database provides a useful perspective, especially for the purposes of the initial database design.

I will illustrate the general principles of this model with the LIBRARY database example, which I will carry through the entire book. This example database is designed to hold data about the books in a certain library. The amount of data we will use will be kept artificially small—just enough to illustrate the concepts. (In fact, at this point, you may want to take a look at the example database. For details on downloading it from the Internet, or on using Microsoft Access to create it yourself, see Appendix D .) In the next chapter, we will actually implement the entity-relationship (E/R) model for our LIBRARY database.

What Is a Database?

A database may be defined as a collection of persistent data. The term persistent is somewhat vague, but is intended to imply that the data has a more-or-less independent existence or that it is semipermanent. For instance, data stored on paper in a filing cabinet, or stored magnetically on a hard disk, CD-ROM, or computer tape is persistent, whereas data stored in a computer’s memory is generally not considered to be persistent. (The term permanent is a bit too strong, since very little in life is truly permanent.)

Of course, this is a very ...

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