E. F. Codd. A Database Sublanguage Founded on the Relational Calculus. In E. F. Codd, A. L. Dean, editors, Proceedings of 1971 ACM-SIGFIDET Workshop on Data Description, Access and Control, San Diego, California, November 11-12, 1971. pages 35-68, ACM, 1971. [doi]
Abstract is missing.