Query processing method for multiple databases with a different set of attributes

This paper discusses a schema integration and query processing method that enables integrated access to the multiple autonomous databases in the distributed environment with schemata restricted to a set of attributes. This method is based on the construction of a virtual database (VDB) formed over physically distributed component databases. Although user queries are issued to this VDB, these queries are decomposed to the subqueries using the virtual functional dependency (VFD), which is defined on the VDB and issued to each actual component database. The VDB-based method can obtain more answers that are logically valid than those gained from conventional query processing methods. When the VFD formed over a VDB is acyclic among the nodes to which the query refers, sound and complete answers can be efficiently calculated.