I am not a database person nor have I used many, however I have been aware of them for a long time.  The following might include prior art:

software was released in 1976:

http://en.wikipedia.org/wiki/Multics_Relational_Data_Store
http://web.mit.edu/multics-history/source/Multics/doc/info_segments/mrds.info

Additional reverences can be found by google for multics mrds