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A new interpretation for null values in the weak instance model

1990; Elsevier BV; Volume: 41; Issue: 1 Linguagem: Inglês

10.1016/0022-0000(90)90032-g

1090-2724

Paolo Atzeni, Maria Cristina De Bernardis,

Data Management and Algorithms

A new definition of the weak instance model for relational databases is presented, which does not consider the missing values as existent though unknown, but just assumes that no information is available about them. It is possible to associate with the new definition logical theories that do not contain existentially quantified variables. The new model enjoys various desirable properties of the classic weak instance model, with respect to dependency satisfaction, query answering, and associated logical theories.