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Definition of External Schemas in Object Oriented Databases

José Samos Departament de Llenguatges i Sistemes Informàtics Universitat Politècnica de Catalunya Pau Gargallo 5, 08028 Barcelona, Spain Winterthur Seguros División Informática Numància 164, 08029 Barcelona, Spain E-mail: jsamos@lsi.upc.es Fax: +34-3-2901551

ABSTRACT: In this work, a new external schema definition methodology for object oriented databases (OODB) that solves some of the problems of existing methodologies is presented. In particular, this new methodology avoids the need for generating derived classes that are not directly used in the defined external schema. Also, it offers a way of integrating and reusing different external schema definitions. KEY WORDS: Object oriented databases, views, external schemas, class integration.

1. Introduction The three-level schema architecture (ANSI/X3/SPARC, 1975) represented in figure (1) has been widely applied in relational databases. In OODB the conceptual and internal schemas have also been studied deeply; this is not the case for external schemas.

External Schema 1 External Schema 2 Conceptual Schema Internal Schema External Schema N

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Figure 1. The ANSI/X3/SPARC Three-Level Schema Architecture. In this paper, the study of external schemas in OODB will be broadened, in particular, external schema definition. A new external schema definition methodology that simplifies considerably the process of definition and the results obtained will be presented. The concepts presented here don't refer to any particular object oriented model; they are general concepts applicable to most existing object oriented models.

1.1 External Schemas and Views A "view" is a simplifying abstraction of a complex structure. Some authors identify the term view with the concept of external schema; others, consider it just as a derived class. In order to avoid confusion, in this work the term view will not be used; instead the terms external schema or derived class...