Schema Integration and Query Processing forMultiple

In a multiple database system, a global schema created by integrating schemas of the component databases provides a uniform interface and high level location transparency for the users to retrieve data. The main problem for constructing a global schema is to resolve connicts among component schemas. In this paper, we deene corresponding assertions for the database administrators to specify the semantic correspondences among component object schemas. Based on these assertions, integration rules are designed, which use a set of primitive integration operators to restructure the component schemas for resolving the connicts and do the integration. The principle of our integration strategy is to keep the data of component databases retrievable from the global schema without losing information. Moreover, more informative query answers may be derived from the multiple databases due to schema integration. The strategies for processing the global queries are proposed, which use the provided mapping information between global schema and component schemas to decompose the global queries into a set of subqueries. A Flow Control Language is then deened to specify the execution ow of the subqueries as well as the integration of the partial results. Some query optimization techniques are considered in the speciication of the execution ow.