Abstract: A classification engine provides flexible support for manipulation of attribute-based data by dynamic generation of SQL with classifiers constructed from different schema objects representing different database schemas. The classifiers may be constructed by defining classifiers corresponding to the database schema, and mapping the classifiers to columns on tables in the database. The invention also allows a classification system to modify the database structure and easily conform the classification engine to the modified structure without recompiling the engine or rewriting the application that use the classification system. The engine is conformed to the new structure by constructing a second schema object for the modified database. The schema objects are preferably defined using a field-based language such as extensible markup language (XML).