does A-->B for the given table

Question

Consider the table:-

A         |  B 

a         |  2

b         |  a

c         |  4

NULL  |  6

NULL  |  6

determine whether F.D. holds for A-->B  and/or B-->A?and How?

Comments

i think a->b will hold while b->a will not hold . because every null is unique in sql . read it in group by clause . so a->b will hold . but in b->a for 6 both null be different . what is the answr??

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Just question is available.

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then don't trust me fully. null is undefined behaviour i m not 100% sure . but in group by clause every null is treated uniquely.

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thanx

Answer 1

We can only be sure whether the FD does not  hold and we can never be sure whether the FD can hold as the values may be differ after insertion(not for instance). So here in the question in this particular instance, A->B can hold as the values in A are different and if both null values are same then also the value in the corresponding B is same(6) and if the value turned out to be different then no issue. So A->B holds. But B->A doesn't hold cause if both the null values turned out to be different (NULL values do not compare equal in SQL) then it will fail..