Data Modeling with the Entity-Relationship Model - Database Processing: Fundamentals, Design, and Implementation - page 209

EMPLOYEE

EmployeeName

Other Data . . .

Figure 5-41
Data Model for the
Management Structure
in Figure 5-40 as a 1:N
Recursive Relationship Manages

Child's Red
Wagon

Handle
Assembly

Wheel
Assembly

Body

Figure 5-42
Bill of Materials

Handle Bolt Washer Nut Wheel Axle

PART

PartName

Other Data . . .

Figure 5-43
Data Model for the Bill of
Materials in Figure 5-42 as an
N:M Recursive Relationship

By ThE Way What would happen to the data model if the diagram showed how many
of each part are used? Suppose, for example, that the wheel assembly
requires four washers and the handle assembly requires just one. The data model in
Figure 5-43 will not be correct for this circumstance. In fact, adding Quantity to this
N:M relationship is analogous to adding Price to the N:M relationship in Figure 5-22.
See Project Question 5.63.

N:M recursive relationships can be used to model directed networks, such as the flow of docu-
ments through organizational departments or the flow of gas through a pipeline. They also can be
used to model the succession of parents, in which mothers, fathers, and stepparents are included.
If recursive structures seem hard to comprehend, don't fret. They may seem strange at first,
but they are not difficult. Work through some data examples to gain confidence. Make up a train
and see how the model in Figure 5-38 applies, or change the example in Figure 5-40 from employ-
ees to departments and see how the model in Figure 5-41 needs to be adjusted. Once you have
learned to identify recursive patterns, you'll find it easy to create models for them.

The Data Modeling Process

During the data modeling process, the development team analyzes user requirements and
constructs a data model from forms, reports, data sources, and user interviews. The process is
always iterative; a model is constructed from one form or report and then supplemented and