On semantic issues connected with incomplete information databases
ACM Transactions on Database Systems (TODS)
Extending the database relational model to capture more meaning
ACM Transactions on Database Systems (TODS)
On Databases with Incomplete Information
Journal of the ACM (JACM)
A relational model of data for large shared data banks
Communications of the ACM
Principles of Database Systems
Principles of Database Systems
Computers and Intractability: A Guide to the Theory of NP-Completeness
Computers and Intractability: A Guide to the Theory of NP-Completeness
Null values in data base management a denotational semantics approach
SIGMOD '79 Proceedings of the 1979 ACM SIGMOD international conference on Management of data
The relational model of data and cylindrical algebras
PODS '82 Proceedings of the 1st ACM SIGACT-SIGMOD symposium on Principles of database systems
The VLDB Journal — The International Journal on Very Large Data Bases
Towards a logical reconstruction of a theory for locally closed databases
ACM Transactions on Database Systems (TODS)
Describing and deriving certain answers over partial databases
Journal of Intelligent Information Systems
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Precise conditions are formulated which should be satisfied in a semantically meaningful extension of the usual relational operators of projection, selection, union and join, from operators on relations to operators on tables with "null values" of various kinds allowed. These conditions require that our system be safe in the sense that no incorrect conclusion is derivable by using a specified subset Omega of the relational operators; and that it be complete in the sense that all valid conclusions expressible by relational expressions using operators in Omega are in fact derivable in our system. Examples of three such systems are studied in more detail. The effect of dependencies of various kinds is briefly discussed.