摘要
Within integrated multiple object databases, missing data occurs due to the missing attribute conflict as well as the existence of null values. A set of algorithms is provided in this paper to process the predicates of global queries with missing data. For providing more informative answers to users, the maybe results due to missing data are presented in addition to the certain results. The local maybe results may become certain results via the concept of object isomerism. One algorithm is designed based on the centralized approach in which data are forwarded to the same site for integration and processing. Furthermore, for reducing response time, the localized approaches evaluate the predicates within distinct component databases in parallel. The object signature is also applied in the design to further reduce the data transfer. These algorithms are compared and discussed according to the simulation results of both the total execution and response times. Alternately, the global schema may contain multivalued attributes with values derived from attribute values in different component databases. Hence, the proposed approaches are also extended to process the global queries involving this kind of multivalued attribute.