db/journals/jacm/ImielinskiL84.html Tomasz Imielinski Witold Lipski Jr. Incomplete Information in Relational Databases. 761-791 1984 31 J. ACM 4 http://doi.acm.org/10.1145/1634.1886 db/journals/jacm/jacm31.html#ImielinskiL84 JACM/31/P0761.pdf