In this chapter, we investigate the situation where a user’s request for data cannot be answered in the desired way because the data base does not contain the data requested. We argue that in such a situation it is not sufficient to report to the user the fact that the query failed, without providing the deeper reasons for the failure; an informative answer to a failing query, however, will adjust the user’s wrong assumptions about the contents of the data base and often will free him/her from the need of asking additional queries. We describe a formal method by means of which the reasons for the failure of a user’s query expressed in a relational calculus based language may be inferred and reported to the user as answer; special attention is paid to the amount of computation required for this answer generation procedure. We then show that a wide class of integrity constraints of a data base may be used to further improve the inference process and that any abuse of informative answers for the deduction of confidential information may be prevented easily.
KeywordsIntegrity Constraint Confidential Information Original Query Closed World Assumption Qualification Expression
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