1. A method for providing access to aggregate data values, comprising: providing an abstract data layer comprising a set of logical fields for composing an abstract query, wherein each logical field provides an access method that specifies at least a method for accessing data corresponding to the logical field, and wherein the method for accessing data specified by at least one logical field comprises an aggregate access method that specifies a set of input data and an expression for determining an aggregate data value from the set of input data; receiving, from a requesting entity, an abstract query composed from the set of logical fields that includes the at least one logical field specifying an aggregate access method; retrieving the set of input data specified by the aggregate access method; calculating at least one aggregate data value from the set of input data according to the expression specified by the aggregate access method.