The relational model, which SQL draws much of its conceptual core from, was first formally defined in 1970 by Dr. E. F. Codd, a researcher for IBM, in a paper entitled A Relational Model of Data for Large Shared Data Banks . This article generated a great deal of interest in both the feasibility and practicality of such a system in a commercial application.

In 1974 IBM began the System/R project, and with the work of Donald Chamberlin and others, defined SEQUEL, a Structured English Query Language . System/R was implemented on an IBM prototype called SEQUEL-XRM in 1974-75. System/R was then re-written completely from 1976-1977 in order to implement multi-table and multi-user features. As the system was revised, it was briefly re-named "SEQUEL/2", before eventually being re-named to "SQL" for legal reasons.

1978 saw the beginning of the methodical testing of System/R at customer test sites. Demonstrating both the usefulness and practicality of the system, this testing proved to be a success for IBM. As a result, IBM began to develop commercial products that implemented SQL based on their System R prototype, including SQL/DS introduced in 1981, and DB2 in 1983.

Several other software vendors accepted the rise of the relational model, and announced SQL-based products. These included Oracle (who actually beat IBM to market by two years by releasing their first commercial RDBMS, in 1979), Sybase, and Ingres (based on the University of California's Berkeley Ingres project).

Note Ingres and PostgreSQL
 

PostgreSQL's name is, as you might have guessed, a play on the name Ingres. Both PostgreSQL and Ingres trace their roots back to the UC Berkeley's Ingres RDBMS system.