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+19 votes

Form the following instance of a relation schema R(A,B,C), we can conclude that:

1 1 1
1 1 0
2 3 2
2 3 2
  1. A functionally determines B and B functionally determines C
  2. A functionally determines B and B does not functionally determine C
  3. B does not functionally determine C
  4. A does not functionally determine B and B does not functionally determine C
asked in Databases by Veteran (68.8k points) | 1.4k views

before solving read carefully 
"Form the following instance of a relation schema R(A,B,C)"
Relation = one table
Instance = values at one moment ( as opposed to the schema, which is the description of all possible values)


5 Answers

+35 votes
Best answer
Ans. C

Generally Normalization is done on the schema itself.

From the relational instance given,we may strike out FD s that do not hold.

e.g.B does not functionally determine C(This is true).

But we cannot say that A functionally determines B for the entire relation itself.This is because that ,A->B holds for this instance,but in future there might be some tuples added to the instance that may violate A->B.

So overall on the relation we cannot conclude that A->B,from the relational instance which is just a subset of an entire relation.
answered by Loyal (3.7k points)
selected by
+6 votes

Answer - (C)

  • If we observe carefully "Instance" of a relation Schema R(A,B,C) is given here.

Now as we can see A functionally determines B for the present tuples.

But B does not determines C. That is clearly Visible.

  • In future there may be chances of tuples to be present where A can not determine B uniquely.

So,option C is most suitable.

answered by Loyal (3.4k points)
+1 vote

ans is c


answered by (29 points)
–1 vote
When value of A is 1,B is 1.When value of A is 2,B is 3.So A functionally determines B.

When value of B is 1,C is 1 and in another case C is 0.So B does not functionally determine C.

Hence,the answer is B.
answered by (17 points) 1 flag
wrong answer.
–2 votes
Ans will be B

Here A->B satisfies(for same value in A , B also gives unique value)

B->C not satisfies(when B is 1 , C gives two values 1,0)
answered by Veteran (76.3k points)
For the given instance yes, but there can be another instance also for R where the FD may not hold. So, from a given instance we can only say "no FD".

  srestha @Arjun sir

question says ,Form the following instance of a relation so according to that B should be answer .why we are considering other possibilities ?pls clear this 

Suppose a person from religion X throws a bomb. From this can we conclude that religion X is teaching terrorism?

Likewise everything depends on the definition. FD is defined on relational schema and not on any instance alone.

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