conf/pods/82 Umeshwar Dayal Nathan Goodman Randy H. Katz An Extended Relational Algebra with Control over Duplicate Elimination. 117-123 1982 PODS db/conf/pods/pods82.html#DayalGK82 http://doi.acm.org/10.1145/588111.588132 db/conf/pods/DayalGK82.html journals/anthology/cd1-3:PODS82/P117.PDF journals/anthology/dvd1:text/1-3/PODS82/P117.PDF journals/siamcomp/AhoSU79 journals/tods/AhoSU79 ... journals/tods/Chen76 ... journals/tods/Shipman81