Maximal objects and the semantics of universal relation databases

The universal relation concept is intended to provide the database user with a simplified model in which he can compose queries without regard to the underlying structure of the relations in the database. Frequently, the lossless join criterion provides the query interpreter with the clue needed to interpret the query as the user intended. However, some examples exist where interpretation by the lossless-join rule runs contrary to our intuition. To handle some of these cases, we propose a concept called maximal objects, which modifies the universal relation concept in exactly those situations where it appears to go awry—when the underlying relational structure has “cycles.” We offer examples of how the maximal object concept provides intuitively correct interpretations. We also consider how one might construct maximal objects mechanically from purely syntactic structural information—the relation schemes and functional dependencies—about the database.