Four Valued Logic for Relational Database Systems.

http://doi.acm.org/10.1145/382274.382401 db/journals/sigmod/Gessert90.html G. H. Gessert Four Valued Logic for Relational Database Systems. 29-35 sigmodR/19-1/P029.pdf 1990 19 SIGMOD Record 1 db/journals/sigmod/sigmod19.html#Gessert90