Extending the Relational Model Version 2 to Support Ceneralization Hierarchies

Support for generalization hierarchies is widely recognized as a desirable feature of data models. While such hierarchies are indeed supported by data models such as NIAM and object oriënted models, the relational model is still deficiënt in this respect. Proposed enhancements suggested by Smith and Smith in their SHM model and by Codd in his RM/T model have not been included in Codd's new version of the relational model (RM/V2). The present paper proposes an extension to RM/V2 needed to support generalization hierarchies and thus solve database design problems induced by generalization hierarchies. The proposed extension to RM/V2 is first introduced as an extension to a formal model of RM/V2 expressed in RM/V2 itself, i.e. an RM/V2-catalog schema. The paper shows that a small extension to relational language, that uses the extended catalog schema, suffices to solve the acknowledged problems with respect to inquiries on a generalization hierarchy. Next, the paper shows that even in an environment as powerful as SHM or extended RM/V2 there exist inquiries that cannot be implemented easily or at all. It identifies the distinction between data and meta data (or catalog instance data) as the root of this problem. As a powerful solution it then proposes the imploded relational model, in which the distinction between data and meta data has been completely removed, as a conceptual vehicle with which a fundamental solution can be offered.