A new generation of database that integrates knowledge base technology with database systems is presented for providing cooperative (approximate and conceptual) query answering. Based on the database schema and application characteristics, data are organized into type abstraction hierarchies. The higher levels of the hierarchy provide a more abstract data representation than the lower levels. Generalization (moving up in the hierarchy), specialization (moving down in the hierarchy) and association (moving between hierarchies) are the three key operations in deriving cooperative query answers. A relaxation manager is included to provide control for query relaxations. An Explanation System is also included to describe the relaxation process and provide the nearness of the approximate answer. We have demonstrated the feasibility and functionality of CoBase on top of a transportation relational database. The CoBase methodology has also been adopted in the knowledge based multimedia medical distributed database project [5] at UCLA.