The Data Model Resource Book, Volume 3: Universal Patterns for Data Modeling

Silverston c04.tex V2 - 11/21/2008 3:02am Page 138

138 Chapter 4 ■ Hierarchies, Aggregations, and Peer-to-Peer Relationships

Why is i t important to make the distinction between the different ways a

recursive relationship groups data? Understanding the nature of how a class

of data is related helps interpret the meaning and significance of the data

and how you manage that data. Imagine a mobile phone manufacturer that

decides to stop manufacturing a particular model of phone. The design for

the creation of the phone is an aggregation (bill of materials), describing how

the phone is made up of different parts. The phone is no longer manufactured

so this bill of materials becomes obsolete. Even though the complete hierarchy

is now obsolete, each of the parts within the bill of materials may still exist and

be used within other bill of materials for other products.

Now examine the example of the hierarchy of university, schools, and

departments as seen in Figure 4-1. It can be said that the ‘‘Accounting

Department’’ and the ‘‘Business Management Department’’ are members of a

hierarchy called the ‘‘Business School.’’ As we have previously stated, if the

university closes, the schools close and the departments close. In other words if

the ‘‘owning’’ entity of ‘‘XYZ University’’ is destroyed, the hierarchy and each

ofthemembersinthehierarchygetdestroyed.Weshouldbeabletomanage

and understand the different ...