SEMANTIC DATA MODEL TO RELATIONAL DATA MODEL - Database Design and Development: An Essential Guide for IT Professionals - page 292

DepName

EmpId

1 *

SocSecNo

Age

assigned
to

EMPLOYEE

DEPENDENT

EmpName

Relational Notation

EmpAddr

EMPLOYEE (Empld, SocSecNo, EmpName, EmpAddr)

DEPENDENT (Empld, DepName, Age)

Foreign Key: Empld REFERENCES EMPLOYEE

EMPLOYEE relation

EmpId SocSecNo EmpName EmpAddr

1234
2345
3456

101-38-2121
320=55-9641
333-22-1111

Daniel R.Grady 6 William Street, Jamesburg, NJ 08810

Alice M. Knox 1 Bradford Place, Iselin, NJ 08834

Matthew Rader 47 Woods Drive, Edison, NJ 08817

4567 123-47-3257 Ramola Ruskin 55 Walker Lane, Metuchen, NJ 08819

EmpId DepName Age

1234
1234
2345
2345
2345
3456
4567
4567

Mary Grady
John Grady
David Knox
Jeremy Knox
Amanda Knox
Luke Nader
Anne Ruskin
Drew Ruskin

14
11
15
13
12
16
10
5

DEPENDENT
relation

(Weak Entity
Type)

Figure 9-20 Transformation of identifying relationship.

Supersets and Subsets

While creating semantic data models, you discover objects in the real world that are
subsets of other objects. Some objects are specializations of other objects. On the
other hand, you realize that individual object sets may be generalized in supertype
object sets. Each subset of a superset forms a special relationship with its superset.
Figure 9-21 shows the transformation of a superset and its subsets. Note how the
primary key attribute and other attributes migrate from the superset relation to the
subset relations.

OUTCOME OF MODEL TRANSFORMATION

You know the phases that lead up to a relational data model. Initially, you define
the information requirements. Then, you create a semantic data model based on the
real-world information requirements. You may apply either of the two data model-
ing techniques—object-based or entity-relationship modeling—to create the seman-
tic data model. The next step is the important transformation step. You have already
studied the components and concepts of a relational data model. Now the model
transformation accomplishes the creation of the relational data model.
In this section, let us review the model transformation process. We will summa-
rize what has been covered so far in this chapter.