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The principles of queries that allow incomplete answers are presented. Partial answers make it necessary to refine the model of query evaluation. The first modification relates to the satisfaction of conditions, under some circumstances, conditions involving unbound variables as satisfied are considered. Second, in order to prevent a proliferation of answers, only answers that are maximal in the sense that there are no assignments that bind more variables and satisfy the conditions of the query, are accepted. The mode of query evaluation consists of two phases, a search phase and a filter phase. The containment problem for queries consisting only of search constraints under the different semantics are investigated.
Kanza et al. (Sat,) studied this question.