Database Modeling and Design

60 CHAPTER 4 Requirements Analysis and Conceptual Data Modeling
ship between Employee and City, then “lives-in” is not redundant,
because the relationships are unrelated. However, consider the situation
shown in Figure 4.1b for the ER model and Figure 4.1d for UML. The
employee works on a project located in a city, so the “works-in” relationship
between Employee and City is redundant and can be eliminated.
Ternary Relationships
Define ternary relationships carefully. We define a ternary relationship
among three entities only when the concept cannot be represented by
several binary relationships among those entities. For example, let us
assume there is some association among entities Technician, Project,
and Notebook. If each technician can be working on any of several
projects and using the same notebooks on each project, then three
many-to-many binary relationships can be defined (see Figure 4.2a for
the ER model and Figure 4.2c for UML). If, however, each technician is
Technician
N
uses
Technician
Project
N
works-on
1 1
usesnotebook
N
N
Project
N
has
N
Notebook
(a) Binary relationships
Notebook
(b) Different meaning using a
ternary relationship
1
Technician
*
*
works-on
uses
Technician
uses-notebook
* *
Project
*
Project
* *
has
*
Notebook
*
Notebook
(c) Binary associations
(d) Different meaning using a ternary association
Figure 4.2
Ternary relationships