@article{PIM2_1983__34(48)_257662,
     author = {Pavlovi\'c, Gordana M.},
     title = {On the operations over relations in the relational model data with two types of null values.},
     journal = {Publications de l 'Institut Math\'ematique. Nouvelle S\'erie},
     pages = {151},
     publisher = {mathdoc},
     volume = {34(48)},
     year = {1983},
     zbl = {0575.68019},
     language = {en},
     url = {https://geodesic-test.mathdoc.fr/item/PIM2_1983__34(48)_257662/}
}