Understanding the implementation of IS-A relations

Generalization/specialization hierarchies (IS-A relations for short) are basic semantic constructs proposed in most information system conceptual models. At the other side of design methodologies, where standard DBMSs are used, and will still be used for several years, there is no explicit representation of these IS-A relations. As a consequence, all the current methodologies include rules through which these semantic constructs are transformed into standard structures. However, it quickly appears that the translation rules proposed are most often incomplete, and sometimes incorrect. This fact has been experienced by many practitioners, who are faced with complex translation problems, but who do not find satisfying help neither in modem text books, nor m CASE tools. The aim of this paper is to analyze IS-A relations in some detail, and to propose a wide range of correct techniques to express IS-A relations into standard constructs. Understanding these techniques has also proved essential in reverse engineering processes. Keywords ER model, conceptual modelling, supert),pe/subtype hierarchy, IS-A relation, schema transformation, implementation, logical design, physical design, reverse engineering.