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http://doi.acm.org/10.1145/983055.983059 db/journals/sigmod/Codd73.html E. F. Codd Understanding Relations. 63-64 sigmodR/05-1/P063.pdf 1973 5 FDT - Bulletin of ACM SIGFIDET 1 db/journals/sigmod/sigmod5.html#Codd73