DATABASE DESIGN 2: DESIGN METHOD - Concepts of Database Management - page 209

Student

StudentNum LastName FirstName DormNum

1253 Johnson Ann 3

1 1662 Anderson Tom

2108 Lewis Bill

2546 Davis Mary 2

2867 Albers Cathy 2

209

2992 Matthew Mark

3011 Candela Tim 3

3574 Talen Sue

Student StudentDorm

StudentNum LastName FirstName StudentNum DormNum

1253 Johnson Ann 1253 3

1662 Anderson Tom 1662 1

2108 Lewis Bill 2546 2

2546 Davis Mary 2867 2

2867 Albers Cathy 3011 3

2992 Matthew Mark

3011 Candela Tim

3574 Talen Sue

FIGURE 6-27 Student table split to avoid use of null values

In the process, you have created what is formally called an entity subtype . You can say that the
StudentDorm table is a subtype of the Student table. In other words, “students living in dorms” is a subtype
(or subset) of “students.”
Some design methods have specific ways of denoting entity subtypes, but it is not necessary to denote
entity subtypes in DBDL. You can recognize entity subtypes by the fact that the primary key is also a foreign
key, as shown in Figure 6-28.

Studentƒ(StudentNum, LastName, FirstName)

StudentDorm (StudentNum, DormNum)
ƒƒƒƒƒFKƒ ƒStudentNumƒ

→

Student
ƒƒƒƒƒFK ƒƒDormNum → Dorm

FIGURE 6-28 Sample DBDL with entity subtypes

Most approaches to diagramming database designs have ways of representing subtypes. In IDEF1X, for
example, a subtype, which is called a category in IDEF1X terminology, is represented in the manner shown in
Figure 6-29. The circle is the symbol for category. The single line below the circle indicates that the cat-
egory is an incomplete category ; that is, there are students who do not fall into the StudentDorm category.