8. A recommendation system comprising: a storage module configured to store an ontology database comprising an ontology hierarchy structure with N hierarchy levels, N is an integer, each of the hierarchy levels comprises at least one entity, the ontology database is configured to store a plurality of j th user data respectively corresponding to a plurality of users, and each of the j th data records at least one j th entity of the entities on a j th hierarchy level of the ontology hierarchy structure; a converting module configured to generate a plurality of k th user data corresponding to the users according to the j data respectively, wherein each of the k th user data records at least one k th entity of the entities on a k th hierarchy level of the ontology hierarchy structure; a clustering module configured to cluster the k th user data; and a recommendation module configured to recommend the entities in the ontology database to the users according to the clustering result; wherein the converting module is configured to calculate a sparsity of the j th user data, and map the j th entity recorded in each of the j th user data to at least one of the k th entity recorded in each of the k th user data according to the sparsity of the j th user data; wherein a first calculating value is equal to a product of a quantity of the users and a quantity of the entities on the j th hierarchy level, a second calculating value is equal to a quantity of a sum of the j th entity recorded by each of the j th user data divided by the first calculating value, and the sparsity of the j th user data is equal to 1 subtracted by the second calculating value.
12. The recommendation system as claimed in claim 8 , wherein the recommendation module is configured to search a k th frequent entity from the k th entity recorded by each of the k th user data in a cluster, search a j th frequent entity from the entities on the j th hierarchy level according to the k th frequent entity, and recommend the j th frequent entity to one of the users corresponding to the k th user data in the one of the cluster.