conf/vldb/76 J. H. ter Bekke A Data Manipulation Language for Relational Data Structures. 159-168 1976 VLDB db/conf/vldb/vldb76.html#Bekke76 db/conf/vldb/Bekke76.html VLDB76/P159.PDF journals/cacm/Codd70 conf/sigmod/Codd71 persons/Codd72 journals/cacm/AstrahanC75