GATE CSE 2014 | Set 3 | Question: 22

Question

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

• A. in all candidate keys of $R$
• B. in some candidate key of $R$
• C. in a foreign key of $R$
• D. only in the primary key of $R$

Comments

Detailed Video Explanation: https://www.youtube.com/watch?v=HdaHz0iY2ws&t=41s

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when checking for 3NF then we are checking if the FD derives  Prime Attrbute or not if its not the super key , then there Prime Attributes may / may not be present in any CK

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Option B is correct

let R(ABCDEFGH) and ABC is a candidate key then A,B,C is prime attritube

And if ACD is candidate key then D is also a prime attribute rest who are not apper in any candidate key are called no-prime attributes.

 

Answer 1

Answer (B).

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 a prime attribute. And conversely, an attribute that does not occur in ANY candidate key is called a non-prime attribute.

Answer 2

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.

Comments

Of course prime attributes can NEVER be null in Primary key.

Prime attributes are part of Candidate keys that serve the uniqueness feature of tuples. Since any Candidate keys can become the Primary key, therefore prime attributes should not contain NULL values.

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@Rajsukh Mohanty, Your point is not correct. 

Whenever a relation has two or more candidate keys, one of them is arbitrarily selected (by the database designer) and called the primary key of that relation. The entity integrity constraint states that no primary key value can be NULL.

Attributes of the primary key can not be Null, But attributes of other candidate keys may be null.

Saying "prime attribute can not take null values" is Wrong.

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so sir, since we know that Candidate keys can take null values, so, we can say that prime attributes can take NULL values in Candidate keys. right?

Answer 3

Answer is B

Comments

well explained easy explanation thanks 😊

Answer 4

• Candidate Key: A minimal set of attributes that uniquely identify a tuple.

• Prime Attribute: Any attribute that is part of at least one candidate key.

• Non-prime Attribute: Not part of any candidate key.

Given:
Relation R(A, B, C, D, E, F)
Candidate Keys = {A, B}, {C}

So:

• Candidate Key 1: AB

• Candidate Key 2: C

• A, B, and C are prime

• D, E, F are non-prime

• A and B appear in one candidate key (AB), but not in all.C is in a different candidate key.

• Yet all of them (A, B, C) are still prime.

Why "in all candidate keys" is Wrong?

If "prime attribute = in all candidate keys", then:

• In this, only C would qualify (since it’s the only one in its candidate key).

• But that’s wrong — both A, B, and C are prime.

So, prime attribute just means:

Appears in at least one candidate key.

Prime attribute = An attribute that is part of at least one candidate key
It is not necessary for it to be in all candidate keys.

"If an attribute is in even one candidate key, then it's a prime attribute."

Answer 5

COMMENT BELOW IF YOU HAVE ANY DOUBT

Comments

CAN YOU SHARE YOUR  NOTES!

 

Answer 6

Answer: B

A prime attribute should be a part of any one candidate key NOT ALL the candidate keys.

Example: R (A,B,C,D,E)

Let, Candidate keys: AB, BC

Prime attributes: A, B, C

Note: A appears in AB but NOT in BC. Similarly, C appears in BC but NOT in AB.