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Null Values in Definite Programs

出版	University of Maryland, 1990
URL	http://books.google.com.hk/books?id=ZU-D0AEACAAJ&hl=&source=gbs_api

註釋

Abstract: "Null values are a special kind of incomplete information that appear in database applications. There are several kinds of null values. The one [sic] modeled in this paper are those that represent entities that are known to exist but whose exact values are only known to be in a finite subset of constants in a given domain. For example, if we know that Paul is the fraternal grandfather of John, then we know that there is someone who is the father of John and a child of Paul. If we further assume that there are only finitely many individuals, then this someone can be represented by the kind of null value mentioned here. In this paper we incorporate these null values into definite programs by using a new kind of symbols called S-constants