1. A computer-implemented method for answering a database-based query of a computerized database system, said method comprising: generating a canonical individual ind′ with respect to a role and a concept, for an existential restriction in an ontology used in said computerized database system; converting implicit data in original data into explicit data by using said canonical individual ind′ and said ontology; and retrieving an answer meeting all query conditions in a query from said original data and said converted explicit data.