db/journals/jacm/Klug82.html Anthony C. Klug Equivalence of Relational Algebra and Relational Calculus Query Languages Having Aggregate Functions. 699-717 1982 29 J. ACM 3 http://doi.acm.org/10.1145/322326.322332 db/journals/jacm/jacm29.html#Klug82 JACM/29/P0699.pdf