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Beschreibung:
It is claimed that object oriented databases (OODBs) overcome many of the limitations of the relational model. However, the formal foundation of OODB concepts is still an open problem. Even worse, for relational databases a commonly accepted datamodel existed very early on whereas for OODBs the uni,cation of concepts is outstanding. Our research in Hamburg and Rostock is directed towards a formally founded object oriented datamodel (OODM) and to contribute to the development of a uniform mathematical theory of OODBs. This report contains the results of our first investigations on the OODM. A clear distinction between objects and values turns out to be essential in the OODM. Types and Classes are used to structure values and objects repectively. Then the problem of unique object identi,cation occurs. We show that this problem can be be solved for classes with extents that are completely representable by values. Such classes are called valuerepresentable. The finiteness of a database and the existence of finitely representable rational tree types are sufficient to decide value-representability. Another advantage of the relational approach is the existence of structurally determined canonical update operations. We show that this property can be carried over to object-oriented datamodels i, classes are value-representable. Moreover, in this case database consistency with respect to implicitly specified referential and inclusion constraints will be automatically preserved. This result can be generalized with respect to distinguished classes of explicitly stated static constraints. We show that integrity enforcement is always possible. Given some arbitrary method S and some static or transition constraint I there exists a greatest consistent specialization (GCS) SI of S with respect to I. Such a GCS behaves nice in that it is compatible with the conjunction of constraints, inheritance and refinement. For the GCS construction of a user-defined operation, however, it is in general not suffcient to replace the involved ...