The Difficulty of Mapping Modeled Associations to SQL

Entity-Relationship and UML class diagrams allow users to model the structural relationships, i.e., associations, that occur between entities or objects. By extending these diagrams with Object Relationship Notation (ORN), users can model the semantics of a variety of common association types with improved precision. Mapping these semantics to SQL, however, is very difficult. Here, we provide a brief overview of ORN, show with examples the degree of complexity that is still required to implement common association semantics in standard SQL, and propose a solution. Our purpose is to stimulate efforts to improve SQL in order to close the gap between the modeling of associations and their implementation.