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; retrieving an answer meeting all query conditions in a query from said original data and said converted explicit data; and normalizing a concept axiom and a role axiom in said ontology into an atomic concept subsumption axiom sub⊂sup, an intersection concept subsumption axiom sub1 ∩sub2 ⊂sup, an existential restriction left subsumption axiom ∃role.sub⊂sup, an existential restriction right subsumption axiom sub⊂∃role.sup, a role subsumption axiom role1 ⊂role2, and a role chain subsumption axiom role1 °role2 ⊂role3, wherein sub represents a subclass concept, sup represents a superclass concept, and role represents a role; wherein said step of converting implicit data in original data into explicit data by using said canonical individual ind′ and said ontology comprises: generating triple data with respect to said original data and said canonical individual ind′ based on said normalized concept axiom and said role axiom using said original data and said canonical individual ind′ and a corresponding role and concept.
6. The method according to claim 1 , wherein said step of converting implicit data in original data into explicit data by using said canonical individual ind′ and said ontology further comprises: converting said original data into data in the form of existential restriction right subsumption EXISTSSUB (sub, role, sup) according to said normalized existential restriction right subsumption axiom sub ⊂ ∃role.sup; storing data of a membership triple in the form of instance type TYPEOF(ind, concept), and storing data of a relationship triple in the form of instance role RELATIONSHIP(ind1, role, ind2), wherein said TYPEOF(ind, concept) represents that an individual ind is a type of a concept concept, said RELATIONSHIP(ind1, role, ind2) represents that individuals ind1 and ind2 have a role relationship therebetween, and said concept concept is a superclass concept sup or a subclass concept sub; storing said generated canonical individual ind′ and said corresponding role and concept in the form of CANONIND(ind′, role, concept); and generating data in the form of instance role with respect to a named individual and a canonical individual RELATIONSHIP(ind, role, ind′) and data in the form of instance type with respect to the canonical individual TYPEOF(ind′, sup), according to said data in the form of existential restriction right subsumption EXISTSSUB(sub, role, sup), data in the form of instance type TYPEOF(ind, sub), and data of said canonical individual ind′ and corresponding role and concept CANONIND(ind′, role, sup).