Deductive Databases with Incomplete Information

We consider query processing in deductive databases with incomplete information in the form of null values. We motivate the problem of extracting the maximal information from a (deductive) database in response to queries, and formalize this in the form of conditional answers. We give a sound and complete top-down proof procedure for generating conditional answers. We also extend the well-known magic sets method to handle null values, and show that the transformed program executed by semi-naive evaluation (with minor extensions) is correct in the sense that it will generate all and only valid conditional answers w.r.t. the original program.