Theoretical Aspects of Schema Merging

A general technique for merging database schemas is developed that has a number of advantages over existing techniques, the most important of which is that schemas are placed in a partial order that has bounded 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 not 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 combined with existing schemas using precisely the same merging operation. The technique is general and can be applied to a variety of data models. It can also deal with certain cardinality constraints that arise through the imposition of keys. A prototype implementation, together with a graphical interface, has been developed.