ISRO CSE 2014 | Question: 30

Question

Every time the attribute A appears, it is matched with the same value of attribute B but not the same value of attribute C. Which of the following is true?

• A. A -> (B,C)      
• B. A -> B, A ->> C      
• C. A -> B, C ->> A        
• D. A ->> B, B -> C

Answer 1

the option b is correct . there is just a small definition of multivalue dependencies which . if a->> c is a dependency it means for a ,c has more than one value.

here it is saying whenever a appears b has same value which is true in case of functional dependency . if a->b exist for every a if repeats there should be same b . so a-.b exist here and it is clearly saying c is not same which means for same a it contains multipule value of c. so answer is b .
why not c ??

just consider the case
a is 1

and for c

A       C

1       3

1        4

now u can see that for a we have multipule value of c but c does not have multiple values for a . so we cannot say c->>a . but we can say a->>c. and question is saying c can take different value not a .

Comments

i did not get ...y c should not be answer..? plz explain  y not  c ????

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because c->a is a functiuonal dependency

for multivalued dependency, for same value of LHS we shall get diff value of RHS which is NOT the case of fucntional dependency

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A	B	C
1	20	2
1	20	3
2	4	1
3	5	1

here, A→ B (because, if T1(A) = T2(A) = 1 then, T1(B) = T2(B) = 20 which is true)

But, A→ C (NOT true because T1(A) = T2(A) = 1 then, T1(C) != T2(C) )

so, A -» C

Also, A→ B,C (false)  because T1(A) = T2(A) = 1 then, T1(BC) != T2(BC)