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BCNF is not used for cases where a relation has

1. Two (or more) candidate keys
2. Two candidate keys and composite
3. The candidate key overlap
4. Two mutually exclusive foreign keys
| 2.9k views

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

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

by Veteran (117k points)
0
what is the issue for B?
0
Candidate keys are composite

then there is a chance of partial dependency

if partial dependency is there , then relation is not in 2NF
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Candidate keys are composite then there is a chance of partial dependency; It is okay. If there is no partial dependency then no problem to take composite key as case for BCNF. So I think except D option all are cases for BCNF.

What about the option D, I am not getting meaning of this?
+3

Composite Key is a key which is combination of more than one field or column of a given table. It can be a Candidate key, Primary key. So, a table with combination of its primary key and candidate key can be called as composite key.

Let R(A,B,C,D)

AB->CD and AC->BD. These keys are composite as well as candidate and the relation is in BCNF.

I think the answer must be d by option elimination but i don't really get the meaning of it.

http://sql-plsql.blogspot.in/2013/06/difference-candidate-composite-key.html

+9
only the question setter can answer this :O
+2
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.