can someone eaplain this question Normalization closed

Question

Which of the following statement false is relation R is in 3NF but not BCNF?

A) Relation R must consist atleast two over-lapped candidate keys.

B) Relation R must consist proper subset of candidate key determines proper subset of some other candidate key.

C) Relation R must consist atmost one compound candidate key and other candidate keys simple candidate key.

D) Relation R must consist atleast two compound candidate keys.

Comments

duplicate: https://gateoverflow.in/112957/made-easy

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that question in link has no answer...

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is it the answer given opton b??

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@pranjal.. is this you? from my class?cucse? :p

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ans given is C

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yes ..from your class :P

Answer 1

Compound key means it is a set of superkeys that is not minimal.. A relation in BCNF if X-->A X is in superkeys.. option C says atmost one compound key it may or may not be present..if present the relation is in BCNF also..thats why statement C false here.option B is in always 3NF..

Comments

Only in rare cases does a 3NF table not meet the requirements of BCNF. A 3NF table that does not have multiple overlapping candidate keys is guaranteed to be in BCNF.^[5]Depending on what its functional dependencies are, a 3NF table with two or more overlapping candidate keys may or may not be in BCNF.

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Option B, C are False.

If anyone thinks that option B is True, then it is MISCONCEPTION. MUST Watch This: Misconception in 3NF But Not BCNF 

Answer 2

Consider the following 3 examples which are in 3NF but not in BCNF.and validate the options

Ex1: R = {A, B, C, D, E} and F = {A -> B, B C - > E, E D -> A}.

We get Candidate keys = {ACD, BCD, CDE}  and Prime Attributes = A, B, C ,D, E(all attributes)

Ex2: R(ABC) and F = {AB -> C, C - > B}. 

We get Candidate keys = {AB,AC}  and Prime Attributes = A, B, C (all attributes)

Ex3: R = {A, B, C, D, E} and F = {AB -> CDE, D -> A}.

We get Candidate keys = {AB, BD}  and Prime Attributes = A, B, D

1. R must contain at least two overlapped CK – This is True. 
2. R must consist proper subset of CK determines proper subset of some other CK- This is true. Every given FD is a proper subset of every other CK.
3. R must consist  at most one compound CK and others are simple CK – This is false. 
4. R must consists atleast two compound CK – This is True. 

Therefore, Only Option C is false.

Comments

Option B, C are False.

If anyone thinks that option B is True, then it is MISCONCEPTION. MUST Watch This: Misconception in 3NF But Not BCNF