A SQL Minute: The Rest Is History
From the introduction to a book that I am writing called Practical SQL Using Oracle Database 11g:
Relational database theory debuted in 1970 in a paper published by IBM researcher Ted Codd titled “A Relational Model of Data for Large Shared Data Banks” which used a mathematical term relation to describe what we call a table. In the paper, Codd described his relational algebra, a “procedural” technique of specifying how to extract the required data, involving the writing of an expression that specifies how to maninpulatemanipulate the data in one or more relations using operations such as restriction, projection, union, difference, and join. In 1972, Codd published another paper titled “Relational Completeness of Data Base Sublanguages” in which he described his relational calculus—a “non-procedural” technique of specifying the properties of the required data, involving the writing of a truth-valued expression using the connectives AND, OR, and NOT and the “existential” and “universal” quantifiers EXISTS and FORALL—and proved that his relational algebra was “complete” in the sense that any relational calculus expression could be translated into an equivalent relational algebra expression. In 1974, IBM researchers Don Chamberlin and Raymond Boyce presented “SEQUEL: A Structured English Query Language” which was also complete in that sense and, a few years later, a tiny little company called Software Development Laboratories led by a certain Larry Ellison was the first out of the gate with a commercial implementation. Software Development Laboratories later became Oracle Corporation and the rest, as they say, is history.