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+22 votes

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 $R$
asked in Databases by Veteran (115k points) | 2.4k views
Why some?  Why not all?
check below comment.
I have a different doubt what if candidate key is= A then is A a prime attribute.??
if FD : A->BC over R(A,B,C) ,

CK : A

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

3 Answers

+29 votes
Best answer

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.

answered by Boss (11.6k points)
edited by
@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

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.


@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
AC→A [ Trivial ]

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

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.
Got it :)
nice explanation satisfy with your explanation.
Can Prime attribute be NULL in any of the the Candidate key ??

Can Prime attribute be NULL in Primary key ??
+6 votes
Answer (B).

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.
answered by Loyal (9.3k points)
–6 votes

(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

answered by Active (3.3k points)
it need not be  in all candidate keys of R

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