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A primary key for an entity is

  1. a candidate key
  2. any attribute
  3. a unique attribute
  4. a superkey

3 Answers

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Option B) is surely wrong.

Option D) you can discard, because in the option candidate keys is there, which is a minimal set of super key.

Now in option (A) and (C), I will choose Option A, because one of the most important properties of Primary key is that it should not have NULL value. But in Option C it can not be gurantee. Hence I will choose Option (A) as answer.
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1. Every primary key is candidate key, and a super key

2. One of the candidate key is chosen as primary key,

example R(A,B,C,D,E,F)

suppose candidate key =AB, CD, F

now anyone of these three can be primary keys

3. any set of attribute which contains candidate key or primary key is super key

example   ABE, FE, ABCDEF like these are super keys

 

Now if a attribute is unique, that it can uniquely identify each row then that attribute can be a candidate or primary key.

example

A   B

1    q

2    r

3   s

3   t

here A can not be a key, because values in A can not uniquely identify each row in B

A(3) = [s,t], on other hand column B can be unique attribute

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