A Unified Treatment of Null Values using Constraints
A Unified Treatment of Null Values using Constraints
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Date
1998-10-15
Authors
Candan, Kasim S.
Grant, John
Subrahmanian, V.S.
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Abstract
An important reality when studying relational databases is the fact
that entries in relational tables may often be "missing" or only partially
specified. The study of such missing information has led to a rich body
of work on "null values." It was recognized early on that there are many
different types of null values, each of which reflects different
intuitions about why a particular piece of information is missing.
Different relations (or even the same relation) could contain different
types of null values; yet, very little work has been done on providing a
unifying model that reasons with different types of nulls. In this paper,
we use constraints to provide a unifying framework for the most common
types of nulls. We show how viewing tuples containing null values of these
types can be viewed as constraints, and how this leads to an algebra for
null values. In particular, this algebra contains a unique operator
(called the "compaction" operator) used to remove redundancies from null
valued relations. We have studied various properties of this algebra. We
have built a prototype implementation based on the null valued operators
described here and conducted various experiments using this testbed.
(Also cross-referenced as UMIACS-TR-95-47)