1. A method for representing incomplete and uncertain information in graph data, the method comprising: receiving 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; and using a computing system running a computer application to: create an incomplete graph data set comprising the plurality of graphs by inserting a variable into at least one of the subject node, the predicate and the object node in one or more graphs within the plurality of graphs; and create an uncertain graph data set from the incomplete graph data set by substituting alternative values for all variables in the incomplete data graph set, 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.