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A prime attribute of a relation scheme $R$ is an attribute that appears

1. in all candidate keys of $R$
2. in some candidate key of $R$
3. in a foreign key of $R$
4. only in the primary key of $R$
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Why some?  Why not all?
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check below comment.
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I have a different doubt what if candidate key is= A then is A a prime attribute.??
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if FD : A->BC over R(A,B,C) ,

CK : A

prime attribute : A
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in any candidate keys is correct..?

Prime attribute is a constituent of a candidate key. it need not present in all candidate keys. Hence, option B is correct.

Correct me if i went wrong.

edited
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@Sankar: yes you are correct it is sufficient if the prime attribute is in any of candidate key. did not read all the options , my bad
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Consider a relation $R(ABCDE)$ and $FD =\left \{ AB\rightarrow CD,C\rightarrow B,D\rightarrow E \right \}$.

Here Candidate key  are $AB$ and $AC$.

Prime Attribute:$A$,$B$,$C$.

Now check is B is appearing in both C.K.?No

So prime attribute appears in some CK.

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@ManojK sir , It wiould be nice if you could explain ( or verify ) how to find the prime attribute

AFAIK ,  Prime attributes are part of CK
Here CK are AB, AC .  Dependncies invloving them
AB→CD
AC→B
AC→A [ Trivial ]

$\Rightarrow$ (Prime attributes are part of Key ) Prime attributes are CD, C,D,B,A

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As you stated Prime attributes are part of CK .

CD is nether CK nor part of any CK here then how CD,D becomes prime attribute.
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Got it :)
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nice explanation satisfy with your explanation.
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Can Prime attribute be NULL in any of the the Candidate key ??

Can Prime attribute be NULL in Primary key ??

The constituent attributes of a Candidate key or simply the attributes of a candidate key are called the prime attributes. Suppose ABC is one candidate key of a Relation R(ABCDEFGH). Then the attributes A, B and C all are prime attributes. Similarly if ABD is also another candidate key in the same relation R, then D is also the prime attribute. And conversely, an attribute that does not occur in ANY candidate key is called a non-prime attribute.

(A) in all candidate keys of $R$

a prime attribute need not be necessaryly in the primary key alone, though attributes in all the candidate key are considered prime attribute

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it need not be  in all candidate keys of R

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