Database Modeling and Design

42 CHAPTER 3 The Unified Modeling Language (UML)
Student
enrollee
class
Course
location
scheduled
day
meeting
time
Room
Day
Time
Figure 3.5 UML n-ary relationship (parallel to Figure 2.7)
Figure 3.5 illustrates another example of a n-ary relationship. The n-
ary relationship may be clarified by specifying roles next to the participating
classes. A Student is an enrollee in a class, associated with a given
Room location, scheduled Day, and meeting Time.
The concept of a primary key arises in the context of database
design. Often, each row of a table is uniquely identified by the values
contained in one or more columns designated as the primary key.
Objects in software are not typically identified in this fashion. As a
result, UML does not have an icon representing a primary key. However,
UML is extensible. The meaning of an element in UML may be extended
Primary key as a stereotype
Composition example
with primary keys
Car
«pk» vin
mileage
color
Invoice
«pk» inv_num
customer_id
inv_date
1 .. *
LineItem
«pk» inv_num
«pk» line_num
description
amount
Figure 3.6
UML constructs illustrating primary keys