Theoretical aspects of schema merging

A general technique for merging database schemas is developed that has a num-ber of advantages over existiug techniques, the most importaut of which is that schemes are placed in a partial order that has bouuded joins. This means that the merging operation, when it succeeds, is both associative and commutative, i.e., that the merge of schemas is independent of the order in which they are considered- a property uot possessed by existing methods. The operation is appropriate for the design of interactive programs as it allows user assertions about relationships between nodes in the schemas to be considered as elementary schemas. These can be combiued with existing schemas using precisely the same merging operation. The technique is general aud can be applied to a variety of data models. It can also deal with certaiu cardiuality coastraiuts that arise through the imposition of keys. A prototype imphuel~tation, together with a graphical interface, has been developed.