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The Computer Journal 1991 34(3):239-244; doi:10.1093/comjnl/34.3.239
© 1991 by British Computer Society
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Default Databases and Incomplete Information

P. King and C. Small *

Department of Computer Studies, Birkbeck College, University of London, Malet Street, London WC1E 7HX, UK

We present the concept of a default database which comprises a set of facts, a set of deduction rules, and a set of defaults. Defaults define assumptions to be made about information not derivable from the rules and facts. Defaults, by augmenting the information which is a consequence solely of the rules and facts, enable definite responses to queries on the basis of ‘common sense’ assumptions rather than responses of the form ‘unknown’. The augmented information is termed an extension. Although such an extension is self-consistent, in general two or more mutually inconsistent extensions can arise. We characterise the rules and defaults of a database as safe if only one extension can arise for any given set of facts. We give conditions which are necessary and sufficient to ensure safety.

Received July 1989. revised November 1990.

* Author for correspondence

§ Department of Computer Studies, Birkbeck College, University of London, Malet Street, London WC1E 7HX


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