Advanced Database Management System - Tutorials and Notes: Find the candidate keys from the given relation for normalization

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## Find the candidate keys from the given relation for normalization

Question:
Consider a relation with schema R(A,B,C,D) with functional dependencies (FD’s):
BC → A, AD → B, CD → B, AC → D.
Find all the candidate keys of R.
Solution:

Let us find the closure for the left hand side (LHS) attributes of given set of functional dependencies.

Let us take the FD BC A to check whether the LHS BC forms a candidate key or not.

 We know Result = How? Description BC BC Given BC BCA BC → A If we know BC, then we can derive A uniquely as per the reflexivity rule, hence result = BCA BCA BCAD AC → D From the previous step we know attribute A, and by the FD AC → D the result becomes ABCD which is equal to R. Hence, BC is a candidate key

We derived all the other candidate keys in the same way as stated above and given in the table below;
 LHS Closure Due to the FDs Result becomes Description (BC)+ BC → A AC → D = ABC (Reflexive) = ABCD (Pseudo-transitive) The result of (BC)+ is equivalent to R. Hence BC is a candidate key. (AD)+ AD → B = ABD (Reflexive) (AD)+ ≠ R. Hence AD does not form candidate key. (CD)+ CD → B BC → A = BCD (Reflexive) = ABCD (Pseudo-transitive) (CD)+ is equivalent to R. Hence CD forms a candidate key. (AC)+ AC → D AD → B or CD → B = ACD (Reflexive) = ABCD (Pseudo-transitive) (AC)+ = R hence is a candidate key.

BC, AC, and CD are the candidate keys for the given relation.

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