Missing Information (Applicable and Inapplicable) in Relational Databases.

http://doi.acm.org/10.1145/16301.16303 db/journals/sigmod/Codd86.html E. F. Codd Missing Information (Applicable and Inapplicable) in Relational Databases. 53-78 sigmodR/15-4/P053.pdf 1986 15 SIGMOD Record 4 db/journals/sigmod/sigmod15.html#Codd86