A new interpretation for null values in the weak instance model☆

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Abstract

A new definition of the weak instance model for relational databases is presented, which does not consider the missing values as existent though unknown, but just assumes that no information is available about them. It is possible to associate with the new definition logical theories that do not contain existentially quantified variables. The new model enjoys various desirable properties of the classic weak instance model, with respect to dependency satisfaction, query answering, and associated logical theories.

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Previous versions of this paper appeared as “A New Basis for the Weak Instance Model” in the “Proceedings of the Sixth ACM Symposium on Principles of Database Systems, San Diego, 1987,” pp. 79–86, and as “The New Weak Instance Model,” Rapporto IASI-CNR R.190, Rome, 1987. All correspondence should be sent to the first author at the address above. The first author is now with Università di Napoli, and the second with SIP S.p.A.

Copyright © 1990 Published by Elsevier Inc.