Local Nulls In Summarised Mobile And
Distributed Databases
Author(s)
D. Chan & J. F. Roddick
Abstract
The concept and semantics of null values in relational databases has been discussed
widely since the introduction of the relational data model in the late 1960s. With
the introduction of highly mobile, distributed databases, in order to preserve the
accepted soundness and completeness criteria, the semantics of the null value
needs to expand to reflect a localised lack of information that may not be apparent
for the global database. This paper discusses an extension to the notion of nulls to
include the semantics of ‘local’ nulls. The paper introduces local nulls in terms of
amendments to the relational algebra and examines its impact on query languages.
1 Previous research and motivation
Much of the research on the semantics of null values in relational databases dates
back to the 1970s and 1980s [1–7]. The two definitions of nulls as given by Codd
are missing and applicable, and missing and inapplicable [1] and Zaniolo [4] later
proposed a third definition as, essentially, a lack of knowledge about the attribute’s
applicability, or no information.
To handle null values, various logical approaches have been developed. For
example, the commonly-used three value logic includes true, false (often by virtue
of a value’s absence - q.v. the closed-world assumption [8]), and a maybe value
which indicates that the results may be true [9, 10]. A four value logic has also been
proposed which includes an additional truth value, which represents the outcome
of evaluating expressions which have inapplicable values [11, 12]. Approaches to
accommodating null values in practical systems include the work of Motro [13]
who uses the ideas of conceptual closeness fill the vacancies represented by a null
value and Roth et al. who aim to include nulls in NF 2 databases [5]. Null values
have also been studied in relation to schema evolution and integration [14, 15].
Keywords
Related Book
Other papers in this volume
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