C. J. Date. A Note on the Relational Calculus. SIGMOD Record, 18(4):12-16, 1989. [doi]
@article{Date89, title = {A Note on the Relational Calculus}, author = {C. J. Date}, year = {1989}, doi = {10.1145/74120.74122}, url = {http://doi.acm.org/10.1145/74120.74122}, tags = {C++}, researchr = {https://researchr.org/publication/Date89}, cites = {0}, citedby = {0}, journal = {SIGMOD Record}, volume = {18}, number = {4}, pages = {12-16}, }