Database Modeling and Design

26 CHAPTER 2 The Entity-Relationship Model
each of the other two associated entities. It is “many” if more than one
instance of it can be associated with one instance of each of the other
two associated entities. In either case, it is assumed that one instance of
each of the other entities is given.
As an example, the relationship “manages” in Figure 2.6c associates
the entities Manager, Engineer, and Project. The entities Engineer and
Technician
1 usesnotebook
1
1
Project
Notebook
A technician uses exactly one notebook for
each project. Each notebook belongs to one
technician for each project. Note that a
technician may still work on many projects
and maintain different notebooks for
different projects.
Functional dependencies
emp-id, project-name → notebook-no
emp-id, notebook-no → project-name
project-name, notebook-no → emp-id
(a) One-to-one-to-one ternary relationship
Project
1
assignedto
N
1
Employee
Location
Each employee assigned to a project works at
only one location for that project, but can be
at different locations for different projects. At
a particular location, an employee works on
only one project. At a particular location,
there can be many employees assigned to
a given project.
Functional dependencies
emp-id, loc-name → project-name
emp-id, project-name → loc-name
(b) One-to-one-to-many ternary relationship
Figure 2.6
Ternary relationships