HP Integrity NonStop J-Series - Manual (Page 55)

DEVELOPING A DATA BASE
Drawing the Relationship Between Classes of Information
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.
Dependents Employee
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