UGC NET CSE | Junet 2015 | Part 3 | Question: 11

Question

The relation schemas $R_1$ and $R_2$ form a Lossless join decomposition of $R$ if and only if

1. $R_1 \cap R_2 \twoheadrightarrow (R_1-R_2)$
2. $R_1 \rightarrow R_2$
3. $R_1 \cap R_2 \twoheadrightarrow (R_2-R_1)$
4. $R_2 \rightarrow R_1 \cap R_2$
   • A. $\text{i and ii}$ happen
   • B. $\text{i and iv}$ happen
   • C. $\text{i and iii}$ happen
   • D. $\text{ii and iii}$ happen

Comments

Why they have given “->>” in question instead of "->". Can anyone please help me on this?

Answer 1

ans is C 

e.g R1=(a,b,c,d)  R2=(c,d,e,f) 

R1-R2=(a,b)  , R2-R1=(e,f)

R1 ∩ R2   =c,d

 c,d is common to both relation ,now  to be lossless c,d must be a superkey in at least one of the relation R 1 and R 2

acc to option C it is superkey in both relation so it is lossless 

 

Comments

@Sanjay Sharma Sir, whats the meaning of '↠' arrow with double head??

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It represents multi valued dependency.Functional dependencies rule out certain tuples from being in a relation. If A → B, then we cannot have two tuples with the same A value but different B values.Multi valued dependencies, on the other hand, do not rule out the existence of certain tuples.

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what about this question https://gateoverflow.in/124831/cil-2017-q87

which has different answer