I think the main nugget is that they took Codds powerful mathematical system for defining and manipulating relations and created a simpler way to interface with

I think the main nugget is that they took Codds powerful mathematical system for defining and manipulating relations and created a simpler way to interface with it through the English keywords SELECT FROM WHERE. I see a similar thing happening in Elm, as it tries to translate Haskell/ML ideas into simpler concepts without all of the category theory.