[pasc1878]

SQL does not fully implement Relational Algebra or Calculus, which are isomorphic.

See Many of Chris Dates' books and things like Tutorial D which do meet the ALgebra.

SQL is near enough the theory to work and also has had so much effort put into making it work fast, reliably and scale for volume that a new language has too much to overcome even if it can deal with all cases. So it won't be replaced soon.

However 100 years is longer than SQL had been around so a proper relation server could come around, there is just too much uncertainty.