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Nonapplicable nulls. (English) Zbl 0618.68079

A nonapplicable null appears in a relation whenever the value of the corresponding attribute does not exist. In order to evaluate a query on a relation r involving such null values, the information contained in r is represented by a set of null-free instances, then the query on r - expressed in a user-friendly query language (Generalized Relational Calculus) - is translated into a set of queries on the null-free instances.
Conversely, we define the operations on relations with nulls (Generalized Relational Algebra) and we proved an extension of Codd’s completeness theorem.

MSC:

68P20 Information storage and retrieval of data
68P05 Data structures
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References:

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