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5 votes
5 votes

Consider the schema $R (A, B, C, D)$ and the functional dependencies $A\to B$ and $C\to D$.  If the decomposition is made as $R_1(A, B)$ and $R_2 (C, D)$, then which of the following is TRUE?

  1. Preserves dependency but cannot perform lossless join
  2. Preserves dependency and performs lossless join
  3. Does not perform dependency and cannot perform lossless join
  4. Does not preserve dependency but perform lossless join
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3 Answers

Best answer
13 votes
13 votes
Decomposition is dependency preserving.

It is not lossless as there is no common attribute on which two sub relations can be  joined to get original one

So ans is a
5 votes
5 votes
for lossless, after decomposition of relation into 2, if there is any common attribute in the decomposed relations and that attribute is a key in any of the relation then it is lossless - (any decomposition should have a common attribute(s) which is a key in one of the relation)

here no attributes are common in R1 and R2

so this one is not lossless.

for dependency preserving= if we are able to derive the original  dependency from the dependencies of the splited schemas then it is dependency preserving.

here from R1===A->B and from R2====c->D can be derived..

so it is DP

ANS=OPTION A
2 votes
2 votes
option a because before decomposition fd's are equal to after decomposition fd's .but it is not lossless because r1 insection r2=phi
Answer:

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