Normalisation

Question

Consider the following statements

1. Decomposition of a relation into BCNF may not be lossless.

2. If R and S are two relations in BCNF the natural join of R and S is also in BCNF.

which of the following is correct ?

a) only 1

b) only 2

c) Both 1 and 2

d) None of these

I have doubt in first option..please exlain..

Comments

we can guarantee that at least one decomposition which gave loss-less in BCNF

but we can't guarantee that at least one decomposition which gave Dependency preserving in BCNF.

 

What is he answer for 2nd statement, ( i hope, it should be some more specific, i mean they didn't mention that those are having common attribute or not? etc... )

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answer is option (a)...

but in the first statement , we check the decomposition of BCNF , the condition must satisfy:-

1. lossless join

2. Dependency preservation

and then it should satisfy BCNF...

so by going this , how it is possible for a BCNF decomposition may or may not be lossless...

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i think if there is one common attribute in it then it will remain in BCNF for ex R(A,B) and S(B,C) then its natural join will be in BCNF.but i am confused what will happen  if there is no any common attribute ??it will become cross product of two relation but we can say that it is in BCNF??if yes how??if no how??

 

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from where you get this questions, even at least one question is also not standard. ( have ambiguity )

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Made easy workbook questions..

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we check the decomposition of BCNF , the condition must satisfy:-

1. lossless join

2. Dependency preservation

sometimes it may not satisfy Dependency preservation.

 

Made easy workbook questions..

is it extra work book? then don't solve questions from it

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option (a) may be wrong consider the situation R(A,B) and S(B,C) number of tuple in R>>S if we take natural join then it may be possible that some of the tuple of relation R which satisfy A->B get lost due to natural join.so it can't satisfy lossless .correct me if my approach is wrong.

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kkk...@shaik masthan

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@Shaik Masthan

what about 1st statement....is it true?

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@himgta

it is also ambiguity question ( i mean question need to some more specific )

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@Shaik Masthan

I just want to know whether the decomposition in BCNF will always be lossless or it can be lossy sometimes

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Decomposition of a relation into BCNF always be lossless.

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@himgta

if you want you can have a loss-less decomposition always, but note that every decomposition of BCNF doesn't need not to satisfy Lossless property.

R(A,B,C,D) with FD set ={AB → D, C → AD}

i want to decompose it in R₁(A,B,D) R₂(A,C,D), it is in BCNF and Dependency Preservation but lossy

R₁(A,B,D) R₂(A,C,D), and R₃(B,C) ===> BCNF, DP and Loss-less

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@ Shaik Masthan but at least one decomposition of that relation into BCNF always be lossless ,not any particular given decomposition.

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@Prateek Raghuvanshi

yes brother, i am also conveyed that one only.