Discussion
An instance of relational schema R (A, B, C) has distinct values of A including NULL values. Which one of the following is true?
Answer: B
Correct answer: A is not a candidate key
A candidate key must satisfy two conditions: uniqueness and no NULLs. Distinct values alone are not enough - a NULL means the value is unknown, so it cannot be relied on to identify a row, and two NULLs cannot even be compared for equality. Since A contains NULLs it is neither a candidate key nor a primary key.
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