SQL/DS was actually a slightly reworked System R, the research system that invented SQL. And the original DB2 relational layer was reworked from System R. See:

SQL/DS was actually a slightly reworked System R, the research system that invented SQL. And the original DB2 relational layer was reworked from System R. See: https://www.mcjones.org/System_R/SQL_Reunion_95/index.html https://www.mcjones.org/System_R/SQL_Reunion_95/index.html