A system, computer readable storage medium and method for creating re-usable queries over complex data including hierarchies, trees and graphs is described. This is achieved by an object-oriented query language, where a class is a logical property of a data item, and inheritance is implication between such properties. Virtual method calls execute all relevant method implementations in most specific classes. Expressions can be multi-valued, thus avoiding the need for naming intermediate results. All constructs closely follow the syntax of mainstream object-oriented languages like Java.