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Dependency Preservation

Why Do We Preserve The Dependency?. We would like to check easily that updates to the database do not result in illegal relations being created. It would be nice if our design allowed us to check updates without having to compute natural joins. . Definition. A decomposition D = {R1, R2, ..., Rn} o

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Dependency Preservation

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    1. Dependency Preservation By Jason Allen

    2. Why Do We Preserve The Dependency? We would like to check easily that updates to the database do not result in illegal relations being created. It would be nice if our design allowed us to check updates without having to compute natural joins.

    3. Definition A decomposition D = {R1, R2, ..., Rn} of R is dependency-preserving with respect to F if the union of the projections of F on each Ri in D is equivalent to F; that is if (F1 ? F2 ? … ? Fn )+ = F +

    4. In Layman’s Term Each Functional Dependency specified in F either appears directly in one of the relations in the decomposition.

    5. Continue… It is not necessary that all dependencies from the relation R appear in some relation Ri. It is sufficient that the union of the dependencies on all the relations Ri be equivalent to the dependencies on R.

    6. Property of Dependency-Preservation If a decomposition is not dependency-preserving, therefore, that dependency is lost in the decomposition.

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