Updating logical databases containing null values
We show that it is natural to extend the concept of database updates to encompass databases with incomplete information in the form of null values, or tuple attribute values that are known to exist but whose exact value is not known. Our approach embeds the incomplete database and the updates in the language of mathematical logic, which we believe has strong advantages over relational tables and traditional data manipulation languages in the incomplete information situation. We present semantics for our update operators, and provide an algorithm to perform the operations. The computational complexity of the algorithm is examined, and a lazy evaluation scheme coupled with simple user-supplied cost limits is recommended to avoid undesirable expense during execution. This paper extends results presented in [Winslett 86a].
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