Transforming Data Models into Database Designs - Database Processing: Fundamentals, Design, and Implementation - page 242

As shown in Figure 6-13, association entities sometimes connect more than two entity
types. Figure 6-13(a) shows the association relationship among the CLIENT, ARCHITECT,
and PROJECT entities from the data model we created in Figure 5-23. When there are several
participants in the association, the strategy just shown is simply extended. The associa-
tion table will have the key of each of its parents, as shown in Figure 6-13(b). In this case, the
ASSIGNMENT table has three foreign keys and one nonkey attribute, HoursWorked.
In both of these examples, it is only coincidence that the association tables have only
one nonkey attribute. In general, an association table can have as many nonkey attributes as
necessary to meet user requirements.

CLIENT ARCHITECT

Figure 6-13
Transformation of
ID-Dependent Entities in an
Association Relationship
Among Three Entities

ClientName ArchitectName

Email
Phone

Office
Email

ASSIGNMENT
ClientName
ArchitectName
ProjectName

HoursWorked

PROJECT

ProjectName

StartDate
Budget

(a) Association Pattern Data Model from Figure 5-23

CLIENT ARCHITECT

ClientName ArchitectName

Email
Phone

Office
Email

ASSIGNMENT
ClientName (FK)
ArchitectName (FK)
ProjectName (FK)

HoursWorked

PROJECT

ProjectName

StartDate
Budget

(b) Association Pattern Database Design