Wikipedia
Dataphor
Dataphor is an open-source truly- relational database management system ( RDBMS) and its accompanying user interface technologies, which together are designed to provide highly declarative software application development. The Dataphor Server has its own storage engine or it can be a virtual, or federated, DBMS, meaning that it can utilize other database engines for storage.
Dataphor has been praised for its adherence to relational principles, more closely so than any SQL product.