Resolving domain incompatibility among independently developedndatabases often involves uncertain information. DeMichiel (1989) showednthat uncertain information can be generated by the mapping ofnconflicting attributes to a common domain, based on some domainnknowledge. The authors show that uncertain information can also arisenwhen the database integration process requires information not directlynrepresented in the component databases, but can be obtained through somensummary of data. They therefore propose an extended relational modelnbased on Dempster-Shafer theory of evidence (1976) to incorporate suchnuncertain knowledge about the source databases. They also develop a fullnset of extended relational operations over the extended relations. Innparticular, an extended union operation has been formalized to combinentwo extended relations using Dempster's rule of combination. The closurenand boundedness properties of the proposed extended operations arenformulated
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