ISRO2007-54

Question

BCNF is not used for cases where a relation has

• A. Two (or more) candidate keys
• B. Two candidate keys and composite
• C. The candidate key overlap
• D. Two mutually exclusive foreign keys

Comments

A Very Poor Question. Only the question setter can answer this.

Without wasting time & thoughts, Skip & move on.

Answer 1

Answer B)

A) possible for BCNF. As in BCNF in X-> a transition we take all X as super key and it should be in 3 NF

C) possible

Say a relation AB -> CD

                    AC -> BD

                    AD -> BC

All left side are candidate keys and it is in 3NF , So also in BCNF

D) Mutually exclusive foreign keys not a problem for that relational table and also constructing a BCNF

Comments

I think C is more correct than B because overlap between keys can happen only when they are composite.

Ex- $AB\rightarrow C$, $C\rightarrow B $. Here both $AB$ and $AC$ are keys, they overlap in A, and this relation is not in BCNF.

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if this is so , then even A is wrong, btw  if both AB and AC are both candidate key imples prime attributes are {A,B,C} so what restricts it not to be in BCNF

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Yes C is more stricter than B

Answer 2

bcnf is not used meaning... relation can never be in bcnf if this criteria holds..

for A) B) C) we can make numerous examples that can be bcnf....and also numerous examples that can't be..

for D) relation(synonymous to table) has 2 mutually exclusive foreign keys meaning...

say R(abcdef) here a is key...d,f are referring to a ; so both of them are f.key.  now as they are mutually exclusive we always have d-->f which is   non-key ---> non-key

so this can't ever be a bcnf..