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NIELIT Scientist B 2020 November: 96

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An instance of relational schema $R(A,B,C)$ has distinct values of $A$ including $NULL$ values. Which one of the following is true?

  1. $A$ is a candidate key
  2. $A$ is not a candidate key
  3. $A$ is a primary key
  4. Both “$A$ is a candidate key” and “$A$ is a primary key”
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It is candidate key, because it has distinct value but it cant be primary key as primary key doesnt allow null value
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They have mentioned clearly that it’s “An instance of relational schema...”

And based on the instance we cannot conclude if an attribute will be CK or NOT. May be now, at this point of time, the values are distinct but it doesn’t mean that in future also it’s going to be distinct. Also, for the same reason we cannot even conclude that it’s not the CK. It may so happen that it’s the CK. Both are possible.

Similar question asked in GATE:

Databases: GATE CSE 1994 | Question: 3.7 (gateoverflow.in)

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