Implementing Specialisation Hierarchies within Relational Databases

The extended entity relationship model (EERM) attempts to capture more semantic detail than is otherwise possible in the standard entity relationship model. In particular, the specialisation hierarchy, which is conceptually similar to the Object Oriented notion of inheritance, is a construct used in EERM for representing entity supertypes and subtypes. Grouping entities into various types based on the generic and unique characteristics they contain, affords the benefit of avoiding null values in the resulting table structures. The approach also allows relationships that are unique to a particular subtype to be modelled. However, it is the author’s experience that while many course texts provide examples of how these specialisation hierarchies can be logically modelled, little is said on how they can be implemented within relational databases. A technique that has been used with second year IT degree students to map specialisation hierarchies to the internal model of a relational DBMS will be presented