Schema Translation - Information Systems Reengineering, Integration and Normalization - page 82

domain. The UML model can be regarded as an extension of the EER model with
complete object-oriented features, a comprehensive object-oriented database model
enhanced with advanced semantic features. The UML model improves the EER
model in the areas of expressiveness and readability. It is thus reasonable to follow
the traditional method to design a database starting with the EER model for its rich-
ness in static semantic data modeling techniques, and then map it to a UML model
as part of an object-oriented database design.

3.1 Directly Translating a Network Model

into a Relational Model

Translation from a network schema to a relational schema involves a one-to-one
mapping between the record type and the relation. The set structure of the network
schema is translated into the referential relationship between parent and child rela-
tions. For example, Zaniolo ( 1979 ) designed a set of relations that recast the logical
network schema in terms of a relational model as shown in the following procedure:
Step 1—Derive relations.
Map each network record type to a relation in a one-to-one manner.
Step 2—Derive relation keys.
Map each record key of a network schema to a primary key in a relational table.
However, if the existing network record key is not unique, then it is concatenated
with its owner record key in order to create a unique primary key. The owner record
key is also mapped to a foreign key in the relational table to link the parent and
child records. If the set membership in the logical network schema is manual, then
the record key of member record will be mapped as a candidate key in the relational
table. For instance, Fig. 3.1 is the network schema for a US President.
Applying the above steps, we can map the network schema in Fig. 3.1 to the
following relations:

PRESIDENT (Plname, Pfname, Party, Collg, *Sname)
ADMINISTRATION (Adm#, Iny, Inm, Ind, *Plname, *Pfname)
STATE (Sname, Cap, Pln, Pfn, Adm#, Yad )
ELECTION (Eyear, Winvotes, *Plname, *Pfname)
LINK (*Plname, *Pfname, Cngr#)
CONGRESS (Cngr#, Hd, Hr, Sd, Sr)

Note: italicized words are candidate keys, underlined words are primary keys, and
words with “*” prefixes are foreign keys.