A definition tool of the present invention declares columns which are concealed in external tables to a multi-database server. The multi-database server controls a decomposition and an execution of query messages such that the multi-database server does not perform an arithmetic using the concealed columns....http://www.google.de/patents/US6792425?utm_source=gb-gplus-sharePatent US6792425 - Secure multi database system including a client, multi database server, and database server
