Incomplete Information in Relational Databases

@article{Imielinski1984IncompleteII,
  title={Incomplete Information in Relational Databases},
  author={Tomasz Imielinski and Witold Lipski},
  journal={J. ACM},
  year={1984},
  volume={31},
  pages={761-791}
}
ABSTRACT This paper concerns the semantics of Codd's relational model of data. Formulated are precise conditions that should be satisfied in a semantically meaningful extension of the usual relational operators, such as projection, selection, union, and join, from operators on relations to operators on tables with “null values” of various kinds allowed. These conditions require that the system be safe in the sense that no incorrect conclusion is derivable by using a specified subset Ω of the… 
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