1. A computer-implemented method for conducting a database graph query, comprising: (a) obtaining a first database graph and a second database graph, wherein the first database graph and second database graph each have two or more vertices and one or more edges; (b) mapping the first database graph to the second database graph, wherein: (i) each vertex in the first database graph has a corresponding vertex in the second database graph; and (ii) each edge in the first database graph has a corresponding edge in the second database graph; (c) creating a graph closure tree comprised of a union of the first database graph and the second database graph based on the mapping, wherein each node of the graph closure tree comprises a graph closure of the node's children and each child of a leaf node comprises a database graph; and (d) conducting a graph query based on the graph closure tree.