Schema Translation - Information Systems Reengineering, Integration and Normalization - page 81

Chapter 3
Schema Translation
A database system consists of three components: schemas, data, and programs.
Database reengineering starts with the schema, which defines the meaning of data
and their relationship in different models. Only after a schema has been redefined
can data and programs be reengineered into a new database system, which makes
use of the translated schema. Schema translation is the process of changing a sche-
ma expressed in one data model into an equivalent schema expressed in a different
data model.
This chapter describes the techniques of translating the hierarchical model or the
network model into the relational model. It also outlines a methodology for trans-
forming a relational schema into an object-oriented database schema, and an XML
database schema.
Some work has been done to translate directly a hierarchical model or network
model into a relational model. Others translate a logical hierarchical schema or a
logical network schema into a conceptual schema based on the extended entity re-
lationship (EER) model. The EER model is then translated into a logical relational
schema (Elmasri and Navathe 1989 ).
The object-oriented model is becoming very popular; however, there is no such
thing as a standard object-oriented model. Nevertheless, many conceptual models
for object-oriented database systems exist and have been adopted by the industry.
For example, UML, Booch (Booch 1994 ), and Yourdon are some of conceptual
object-oriented models used to design object-oriented databases. We consider it pre-
mature to address direct translation from a relational into object-oriented database.
Instead, we present a method to translate a relational model into a UML model. We
choose UML model because of its similarity with the EER model. One can translate
a relational model into an EER model in a reverse engineering step and then from
EER model to UML model in forward engineering step, which can then be mapped
to a proprietary object-oriented schema.
Record-based relational databases built by using top-down modeling techniques
such as the EER model have been generally used over the past two decades. Orga-
nizations with such record-based databases could seek to reengineer their databases
into object-oriented databases to capture more of the semantics of the application