Fig. 8

Limits of the variable consistency checking. This example query shows a marginal case in which the query is considered correct by the checking algorithm, but nevertheless, it can be deduced that it has no solution. Consider that all classes used in the example are mutually disjoint. For both union patterns, our checking algorithm observed that the class of variable X is the union of the ex:ClassA and ex:ClassC classes. Variable Y is handled in a similar way, and thus, the query is considered correct. However, it can be logically deduced that the query has no solution. The first union pattern denotes that the class of variable X is ex:ClassA only if the class of variable Y is ex:ClassB. Concurrently, the second union pattern denotes that the class of variable Y is ex:ClassB only if the class of variable X is ex:ClassC. However, classes ex:ClassA and ex:ClassC are disjoint, so the query cannot have a solution