Relation Re nement in Object - Relation Data

Object-relation data models represent eeorts in extending object-oriented data models to incorporate higher level constructs for relating objects. Object-relation data models put explicit relations on the same \level" as types, consequently the relation construct should be a modeling construct which is well integrated with inheritance for object types. Most attempts let the relationships be special objects, and incorporate similar inheritance facilities for relation types as for object types. This paper shows another approach which does not promote relationships to objects. This gives rise to a construct we name relation re-nement, which is meant to resemble inheritance, but which have very diierent characteristics due to relationships being values instead of objects. The paper discusses consequences and use of our model of explicit relations for other major requirements to object-oriented databases, like composite objects, ordered data, and versioning.