An instance of relational schema R (A, B, C) has distinct values of A including NULL values. Which one of the following is true?
An instance of relational schema R (A, B, C) has distinct values of A including NULL values. Which one of the following is true?
(a) A is a candidate key
(b) A is not a candidate key
(c) A is a primary key
(d) Both (a) and (c)
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