SEMANTIC DATA MODEL TO RELATIONAL DATA MODEL - Database Design and Development: An Essential Guide for IT Professionals - page 291

EmpId ProjId

SocSecNo

* *

ProjDesc

assigned
to

EMPLOYEE

PROJECT

ProjStart

EmpName

StartDate HrsWorked

ProjCost

EmpAddr

EMPLOYEE relation

PROJECT relation

EmpId SocSecNo EmpName EmpAddr

ProjId ProjDesc ProjStart ProjCost

1234
2345
3456
4567

101-38-2121 Daniel R.Grady 6 William Street, Jamesburg, NJ 08810

DESN DB Design 6/1/2000 200,000

320 = 55-9641 Alice M. Knox 1 Bradford Place, Iselin, NJ 08834

IMPL DB Implementation 12/15/2000 100,000

333-22-1111 Matthew Rader 47 Woods Drive, Edison, NJ 08817

USER User Training 10/22/2000 75,000

123-47-3257 Ramola Ruskin 55 Walker Lane, Metuchen, NJ 08819 TUNE DB Fine Tuning 3/1/2001 50,000

ASSIGNMENT relation

ProjId EmpId StartDate HrsWorked

DESN
DESN
IMPL
IMPL
USER
USER
TUNE
TUNE
DESN

1234
2345
1234
4567
2345
4567
1234
2345
4567

1/1/2001
1/10/2001
6/15/2001
6/15/2001
7/1/2001
8/31/2001
11/1/2001
12/15/2001
1/22/2001

600
650
400
700
450
500
350
350
500

Figure 9-19 Transformation of aggregate object set.

PROJECT relation with no related rows in the EMPLOYEE relation. Again, such
a condition is not needed in a relational model to satisfy any integrity constraints.
Therefore, it is not generally shown in the relational model. In the relational data-
base system, you may create PROJECT rows without creating corresponding
EMPLOYEE rows without violating any integrity constraints.

Aggregate Objects as Relationships

Recall that in relationships, the participating object sets together form an aggregate
object set by virtue of the relationship itself. Let us discuss how such aggregate
object sets are transformed into the components of a relational data model. Figure
9-19 illustrates such a transformation of an aggregate object set ASSIGNMENT.
Note the intersection relation and the attributes shown in this relation. These are
the attributes of the aggregate object set. You will note that the aggregate object
set becomes the intersection relation.

Identifying Relationship

In the discussion of semantic data modeling, we studied identifying relationships. A
weak entity type is one that depends on another entity type for its existence. A weak
entity type is, in fact, identified by the other entity type. The relationship is there-
fore called an identifying relationship.
Figure 9-20 illustrates the transformation of an identifying relationship. Note
especially the primary key attributes of the weak entity type.