Mini Jini Record Keeper (1983 Jini Micro) for the Commodore VIC20 - This isn't exactly a relational database, but it could certainly be considered a full featured database for it's time, especially considering the hardware it was running on! 

You'll need to do a few steps to get it running in an emulator, because it was originally in cartridge format.  Inside the archive there is a readme.txt that explains what to do, along with the full manual in PDF format.  If you want to know more about where this came from, and how it was converted, check out this forum thread over at the VIC20 Denial forum.