>>to RDBMS and SQL

When you discuss DBMS and SQL, there are two different historical paths.
The one you cited is one source for DBMS theory, but SQL traces its origins to QBE, a language developed by IBM for the legal profession during the late 70's or early 80's.

My first contact with DBMS was with ADABAS on an IBM mainframe during the late 70's. It blew my mind when I first used it.
The mainframe licence for ADABAS cost about $90K.
MYSQL, which is free, does more.
How's that for progress?