A Database Sublanguage Founded on the Relational Calculus.

db/conf/sigmod/Codd71.html E. F. Codd A Database Sublanguage Founded on the Relational Calculus. 35-68 SIGFIDET71/P035.pdf 1971 conf/sigmod/71 SIGFIDET Workshop db/conf/sigmod/sigmod71.html#Codd71