Consider a schema R(A, B, C, D) and functional dependencies A → B and C → D.…

2012

Consider a schema R(A, B, C, D) and functional dependencies A → B and C → D. Then the decomposition R₁(A, B) and R₂(C, D) is

  1. A.

    Dependency preserving but not lossless join

  2. B.

    Dependency preserving and lossless join

  3. C.

    Lossless Join but not dependency preserving

  4. D.

    Lossless Join

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Correct answer: A

To determine the properties of the decomposition R₁(A, B) and R₂(C, D), we analyze dependency preservation and lossless join. First, for dependency preservation: the original functional dependencies are A → B and C → D. Since R₁ contains attributes {A, B}, the dependency A → B is directly preserved within R₁. Similarly, R₂ contains {C, D}, preserving C → D. Thus, the decomposition is dependency preserving.\nSecond, for lossless join: a decomposition is lossless if the intersection of the decomposed relations contains a superkey for at least one relation. Here, R₁ ∩ R₂ = ∅ (empty set). Since there are no common attributes between R₁ and R₂, the natural join of R₁ and R₂ will produce a Cartesian product, potentially generating spurious tuples. Therefore, the decomposition is not lossless.\nConsequently, the correct classification is dependency preserving but not lossless join. Option B and D are incorrect because they claim the decomposition is lossless, which contradicts the empty intersection. Option C is wrong as it claims dependency preservation fails.

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