In this process, ontologies can play a critical role. Query relaxation refers to automatic generalization of the given query to better suit user's information need, particularly with the failing query problem: given a query that returns an empty answer, how can one relax the query's constraints so that it returns a non-empty set of tuples? By generalizing a query to capture " neighbouring " information, query relaxation has been an effective means for fully exploit users' information need from database and information retrieval systems.