Rel is a free, open-source, true relational database management system with an advanced query language called Tutorial D.
Relational
https://reldb.org
https://github.com/DaveVoorhis/Rel
https://reldb.org/c/index.php/read/
GB
Academic, Open Source
Java
Apache v2
https://en.wikipedia.org/wiki/Rel_(DBMS)