1. A computer-implemented system for facilitating cross-subsystem queries of a plurality of building automation subsystems, comprising: a first database storing an ontological model for a building automation system (BAS), wherein the ontological model represents hierarchical relationships between BAS object types without storing data values for particular instances of BAS objects; a second database storing information for the plurality of building automation subsystems, wherein the second database comprises at least one record for each of a plurality of BAS objects, wherein the second database stores each record with a projected semantic type string comprising multiple levels of hierarchical type information for each BAS object and based on the ontological model of the first database; and a query engine configured to decompose a cross-subsystem query received from an application into a plurality of subsystem queries using the information of the second database, wherein the query engine stores the plurality of subsystem queries and combines a plurality of responses to the plurality of subsystem queries into a result of the cross-subsystem query, wherein the query engine causes an updated message to be provided to the application originating the cross-subsystem query when the result of the cross-subsystem query changes, wherein the updated message provided to the application originating the cross-subsystem query includes the changed result of the cross-subsystem query and a projected semantic type string associated with the changed result.