Suppose that you create a database for a personal
telephone directory. The database model must record the names, addresses,
and telephone numbers of people and organizations that the user requires.
First
define the entities. Look carefully at a page from a telephone directory
to identify the entities that it contains. The following figure shows
a sample page from a telephone directory. Figure 1: Partial page from a telephone directory
The physical form of the existing
data can be misleading. Do not let the layout of pages and entries
in the telephone directory mislead you into trying to specify an entity
that represents one entry in the book: an alphabetized record with
fields for name, number, and address. You want to model the data,
not the medium.