Lossy join decomposition

Question

Comments

is answer A??

Answer 1

If R is decomposed into R1 and R2 such that it is a lossy join,this means some unwanted records known as spurious records have come in the join of the decomposed relations ..Hence we are not getting the original relation R but a superset of it because of those spurious records..

Let us consider the statements in light of lossless decomposition first and then negate them for the given question which asks about lossy join..

So if a decomposition is lossless , then by definition : R₁  ⋈  R₂  =  R

So  R ∪ (R₁  ⋈  R₂)   =  R ∪ R  =  R

Hence in case of lossy it would not be..

Similarly iv) is not necessarily true for lossy join as R is subset of (R₁  ⋈  R₂) in case of lossy join..

Hence A) should be the correct answer

Comments

Can you please give an counter example for option (iv).

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Right now I m not able to get that..Sorry for that..:)

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if R(a,b,c,d) is decomposed into R1(a,b) and R2(c,d) then option then there join will be a empty set then option A will hold in this case?