Codd, E. F. (2009). "Derivability, redundancy and consistency of relations stored in large data banks". ACM SIGMOD Record. 38 (1): 17-36. doi:10.1145/1558334.1558336.
Codd, E. F. (1970). "A relational model of data for large shared data banks". Communications of the ACM. 13 (6): 377-387. doi:10.1145/362384.362685.
