@article{PIM_1983_N_S_34_48_a23,
     author = {Gordana M. Pavlovi\'c},
     title = {On the {Operations} {Over} {Relations} in the {Relational} {Model} of {Data} with two {Types} of {Null} {Values}},
     journal = {Publications de l'Institut Math\'ematique},
     pages = {151 },
     publisher = {mathdoc},
     volume = {_N_S_34},
     number = {48},
     year = {1983},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PIM_1983_N_S_34_48_a23/}
}