ENABLE User's Guide

DEVELOPING A DATA BASE

Drawing the Relationship Between Classes of Information

A drawing of the relationships that exist between classes of

information often helps you discover problems in these

relationships. Such a drawing is also helpful later when you use

ENABLE to generate an application that accesses the information

in these classes.

Since only two classes of data exist for the personnel data base,

you can determine the relationship between these classes of data

by drawing a diagram similar to the one shown in Figure 3-3.

DependentsEmployee

Has

Figure 3-3. Relationship Between Classes of Information

Once you have drawn this diagram, the next step is to examine the

relationship to determine how many occurrences of one class

relate to occurrences of the other. Several different types of

relationships can exist:

• one-to-one For each occurrence in one class of information,

there is exactly one occurrence in the other

class.

• one-to-many For each occurrence in one class of information,

there are zero or more occurrences in the other

class.

• many-to-many Zero to many occurrences in one class of

information can be associated with zero to many

occurrences in the other class.

Consider the relationship between employee and dependents. For

each employee occurrence, there is one dependent occurrence.

Figure 3-4 illustrates this relationship.

3-4