The classification problem with semantically heterogeneous data

Given a database fed by two alternative data sources using a common but not identical classification criterion, if we are able to state precisely the semantical connection between the two classification systems, we can derive new and more detailed summary data. Therefore, the question whether an aggregate information is derivable or not, is fundamental to a query-processing system. We state a necessary and sufficient condition which leads to a simple procedure for deciding the answerability of a summary query and evaluating it, if answerable. Surprisingly, the condition of derivability is independent of the database instance and is dependent only on the topological properties of the graph modelling the semantical connection of the classification systems adopted.