From Oracle FAQ
A RDBMS (Relational Database Management System) is a type of DBMS in which the database consists of relations and is manipulated by operations based on relational algebra. The RDBMS was invented by a team lead by Edgar Codd and funded by IBM in the early 1970's.
Influenced by Codd's ideas, many SQL-based systems were subsequently developed: Oracle Database, SQL Server, DB2, Sybase, etc.
The SQL model of data is significantly different from the relational model (e.g. SQL uses a multi-set rather than set-based approach and is not logically equivalent to the relational algebra). Informally, SQL DBMSs are still referred to as RDBMSs because the relational model was such an important influence on the origins of SQL
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