A new polyinstantiation integrity constraint for multilevel relations based on the intuitive idea that every entity in a relation can have at most one tuple for every access class is proposed. The consequences of this property and some of its variations are discussed. A core set of properties which should apply to all relations is identified. These are entity integrity, interinstance integrity, subsumption integrity, and polyinstantiation integrity in the sense of PI-FD. Specific models impose additional polyinstantiation constraints. Oakland requires PI-null, Sea View requires PI-MVD, and the new Franconia model requires PI-Tuple-class. Each of these properties appears likely to arise often enough in practice to justify DBMS (database management system) support for its enforcement on a relation-by-relation basis.
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