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For the relation schema R(A, B, C, D, E) with the following FDs

FD1: {A,B,C} → {D,E} 
FD2: {B,C,D} → {A,E} 
FD3: {C} → {D}

I Decomposed R into a set of BCNF relations.
I got candidate keys as {A,B,C}, {B,C,D} respectively.. how do I find the subset of {A,B,C}?

My solution :
FD3 violates BCNF. Decomposing R using FD3.

R1(C,D) with FDs: 
    FD3, 
    CK: {C} 
R2(A,B,C,E) 
    with new FD: {A,B,C} → E (Decomposed from FD1) , 
    CK: {A,B,C} {BC}+ = {BC} 
  • Not a candidate key {ABC}+ = {ABCDE}
  • A candidate key {BCD}+ = {ABCDE}
  • A candidate key Candidate Keys = {ABC}, {BCD}

The result of the decomposition consists of R1 and R2.

  • But I was said there still exists a subset of {A,B,C} I’m unable to find it.
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  • "find the subset of {A,B,C}" & "there still exists a subset of {A,B,C}" are not clear. Use correct technical terms. Use enough words, sentences & references to parts of examples to clearly & fully say what you mean. . Why are you stuck? You just showed that you have an algorithm to decompose.
    – philipxy
    Jan 8, 2022 at 20:05
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    Your "I have these FDs" doesn't make sense. "These are all the FDs that hold"?--Not possible. "These are all the non-trivial FDs that hold"?--Not possible. "These are some FDs that hold"?--Question can't be answered. Find out what a cover is & what the exact conditions are to apply a particular definition/rule/algorithm. To determine CKs & NFs we must be given FDs that form a cover. Sometimes a minimal/irreducible cover. And the set of all attributes must be given. See this answer.
    – philipxy
    Jan 8, 2022 at 20:07

1 Answer 1

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Assuming that the FD are a cover of the functional dependencies of R, you can find a minimal cover of them:

B C -> A
B C -> E
C -> D

From this minimal cover, one can find easily the only candidate key, which is {B, C}.

As you have correctly said, the dependency C -> D violates the BCNF, so we can decompose R in R1, R2:

R1 = {C D}, with the the projected dependency C -> D, and C the only candidate key
R2 = {A B C E}, the projected dependencies B C -> A, B C -> E, and the only candidate key B C

Both R1 and R2 are in BCNF.

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  • 1
    Since B C -> A and B C -> E are in the minimal cover of the dependencies of R, they hold also in R2, since all their attributes belong to R2.
    – Renzo
    Jan 13, 2022 at 23:55
  • These rules are part of the so-called normalization theory. This is described in many good books on databases. There are also some of them free on internet.
    – Renzo
    Jan 14, 2022 at 17:54

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