Object subclass hierarchy in SQL: a simple approach

Reviewer: Marcos P. Sivitanides

The object subclass hierarchy (OSH) notion is a useful way of specifying property and behavior inheritance in a data model; for example, we can state that, since every student is also a person, the STUDENT object is a subclass of the PERSON object. This paper provides principles, guidelines, and examples of how to model and implement the OSH using SQL views on a relational database management system (RDBMS). In the relational approach, since data about a subclass can be distributed among several base tables, retrieval of subclass data involves joining base tables as well as adherence to subclass inclusion constraints. To address these concerns, the author describes how the view creation mechanism of RDBMSs can be used to specify SQL join views of the base tables that represent the OSH. The author points out that this idea could also be applied to other modeling approaches (this paper focuses only on using SQL views to represent the object subclass). Kung's approach consists of first designing the conceptual data model for the application at hand and, after verification, implementing it on an RDBMS. Subsequently, the user identifies all subclass relationships (there are four types) and, based on transformation rules the author gives, implements the OSH by executing SQL CREATE VIEW statements against the base tables. The traditional problems with view updates remain the same with this approach. The author points out, however, that since the defined views are joins over primary keys of underlying base tables, updates are not difficult; he presents rules and guidelines for appropriately modifying insertion, deletion, and updating queries in order to support the OSH representation by views and to maintain the integrity of the database when these operations occur. Performance issues are addressed by promptly indexing the join and other necessary attributes. I found this paper interesting and conceptually appealing. It presents concrete ideas and examples of how to apply current RDBMS technology to implement some concepts from the object-oriented paradigm. It can be a useful reference for all students and practitioners of database design.