IBM's get-it-out-the-door release of partial (if not broken) implementation of Codd's relational-algebra model, which incorporates parts of predicate-logic, set-theory, and more besides. It kicked-off an industrywide multivendor revolution in information storage, processing, and retrieval.
...but thesedays I'm convinced the point of SQL is to have something other programming languages can point-to to deflect any criticisms of being slow or unwilling to adopt new features, being specifically unergonomic, or most of all: being entirely unresponsive to the day-to-day needs of present-day application developers.
