Suppose we do not require that all attributes in the primary key are non-null and instead request that, in every tuple, at least one key (primary or candidate) does not have nulls in it. (Tuples can have nulls in other places and the non-null key can be di?erent for di?erent tuples.) Give an example of a relational instance that has two distinct tuples that might become one once the values for all nulls become known (that is, are replaced with real values). Explain why this is not possible when one key (such as the primary key) is designated to be non-null for all tuples in the relation.
What will be an ideal response?
Let the relation have attributes A and B, each of which is a key. The tuples can be a, NULL and NULL, b. If the ?rst NULL becomes b and the second a then these tuples become the same. If all tuples are non-NULL over the same key, then they must di?er over that key
somewhere and thus they cannot become the same regardless of what is substituted for nulls in other places.
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