In these approaches, it is considered that an ontology and a legacy database already exist. The goal is to create mapping between them, and/or populate the ontology by the database contents.Mappings here are more complex than those in the previous case because different levels of overlap between the database domain and the ontology’s one can be found, and those domains do not always coincide because the modeling criteria used for designing databases are different from those used for designing ontology models [2]. Both mapping approaches above include two processes: (1) mapping definition i.e. the transformation of database schema into ontology structure, and (2) data migration i.e. the migration of database contents into ontology instances. The migration of database instances into ontological instances (individuals), also called ontology population, may be done in two ways [13] : either as a batch process by dumping all the database instances to the ontology repository, or as a query driven process by transforming only the database instances that are the response to a given query, i.e. only the data needed to answer the user’s query are retrieved from the sources.