19. A non-transitory computer-readable storage medium containing a computer-readable code that when read by a computer causes the computer to perform a method for querying incomplete and uncertain information in graph data, the method comprising: creating an incomplete graph data set comprising the plurality of graphs by inserting a variable into each blank subject node, each blank predicate and each blank object node in each graph within the plurality of graphs; creating an uncertain graph data set from the incomplete data graph set by substituting alternative values for all variables in the incomplete data graph, wherein each variable is replaced by a given group of alternative values, each group of alternative values selected from a set of alternative values comprising a union of all subsets of a set of all uniform resource identifiers in the received graph data and all subsets of a set of all literals in the received graph data; receiving a query from a user over graph data comprising a plurality of graphs, each graph comprising a subject node, an object node and a predicate extending from the subject node to the object node, each subject node comprising a uniform resource identifier or a blank subject node, each predicate comprising a uniform resource identifier or a blank predicate and each object node comprising a uniform resource identifier, a literal or a blank object node; identifying certain answers to the query over the graph data, each certain answer comprising a certain value obtained from a given graph in the plurality of graphs that is equivalent for any alternative value of at least one of the subject node, the object node and the predicate of that given graph; identifying potential answers to the query over the graph data by processing the query over the incomplete data set, each potential answer comprising two or more potential values for at least one of the subject node, the object node and the predicate from a given graph; identifying certain potential answers to the query over the graph data using the uncertain graph data set, each certain potential answer having a number of potential values below a prescribed threshold; and communicating the certain answers and the certain potential answers to the user in response to the query.
21. The non-transitory computer-readable storage medium of claim 19 , wherein the step of identifying potential answers to the query further comprises processing the query over the incomplete data set to identify the potential answers, each potential answer comprising a variable in a given graph in at least one of the subject node, the object node and the predicate of that given graph that can yield the two or more potential values.