A Formalisation of Semantic Schema Integration

Several methodologies for the semantic integration of databases have been proposed in the literature. These often use a variant of the Entity-Relationship (ER) model as the common data model. To aid the schema conforming, merging and restructuring phases of the semantic integration process, various transformations have been deened that map between ER representations which are in some sense equivalent. Our work aims to formalise the notion of schema equivalence and to provide a formal underpinning for the schema integration process. We show how transformational, mapping and behavioural schema equivalence may all be regarded as variants of a more general deenition of schema equivalence. We propose a semantically sound set of primitive transformations and show how they can be used to express the transformations commonly used during the schema integration process. We diierentiate between transformations which apply to any instance of a schema and those which require knowledge-based reasoning since they apply only for certain instances. This distinction could serve to enhance the performance of transformation tools since it identiies which transformations must be veriied by inspection of the schema extension. It also serves to identify when intelligent reasoning is required during the schema integration process.