GATE CSE 2011 | Question: 12

Question

Consider a relational table with a single record for each registered student with the following attributes:

1. $\text {Registration_Num:}$ Unique registration number for each registered student
2. $\text {UID:}$ Unique identity number, unique at the national level for each citizen
3. $\text {BankAccount_Num:}$ Unique account number at the bank. A student can have multiple accounts or joint accounts. This attribute stores the primary account number.
4. $\text {Name:}$ Name of the student
5. $\text {Hostel_Room:}$ Room number of the hostel

Which of the following options is INCORRECT?

• A. $\text {BankAccount_Num}$ is a candidate key
• B. $\text {Registration_Num}$ can be a primary key
• C. $\text {UID}$ is a candidate key if all students are from the same country
• D. If $S$ is a super key such that $S \cap \text{UID}$ is $\text{NULL}$ then $S \cup \text{UID}$ is also a superkey

Comments

1. If S is a super key such that S∩UID is NULL then S∪UID is also a superkey.

 does this create some sort of differance  " such that S∩UID is NULL "

what if we have 

If S is a super key then S∪UID is also a superkey. 

will statement stiil be true?

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" If S is a super key then $S∪UID$ is also a superkey."

@Sun Wukong i think , this will still hold ture !

Assume UID is part of the Super Key S

UID $\subseteq$ S ;  S $\cap$ UID = UID 

now S $\cup$ UID will also be a super key

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Option A is  incorrect becz- it may possible some student don't have account.\

 

Answer 1

Answer is (A)

A relation is given (Registration_Num, UID, BankAccount_Num, Name, Hostel_Room).

Now, Registration_Num is unique for each student. So with this, we can identify each student. Hence, this can be the primary key.

UID: It's an identification number for a person in a country. $($Say you're in India and your UID is $0243.$ Someone in Pakistan may also have the same UID as $0243).$ So, if all students are from India (that is, the same country) then their UID will be different and then UID will be a Candidate key.

If S is a super key then S $\cup$ UID will be a Super key. e.g.  R(A, B, C, D), If AB is a superkey then ABC, ABCD are also superkey.

BankAccount_Num is not a candidate key, because a student can have multiple accounts or joint accounts. We can not identify each student uniquely with BankAccount_Num.

Comments

@tusharp-Nice catch man :). I failed to recognize this earlier.Also, it can be the case the I have a bank account at Union Bank and you have at State Bank, then our account number will be unique at the our respective banks only according to this question(However it will be unique as per financial rules of our country), but since, question requires you to consider only what is given, you can think that two bank accounts in different bank can have same account number. Then also BankAccNum cannot be CK.

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@Ayush Upadhyaya 

consider 2 Brother’s (B1,B2) -→ have One Joint a/c -→ 455XXX

lets take, the both are in same school so, in the database there name,reg…. etc store

also there Bank A/C no store → B1 ---- 455XXX

                                                         B2 ---- 455XXX

so, we can’t take Bank A/C as candidate key….

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ok

 

Answer 2

Ans is A

can have possible duplicate values

As in case of joint account maybe 2 students  having joint account together.

In d) any non key attribute combined with superkey can uniquely identify all attributes.

Comments

ok tnks

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what happed if  students are not registered ?

@Digvijay Pandey

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@mohan123 We don’t have to apply any extra knowledge from what we know generally, we need to consider only what is given in the question. In the question, it is clearly mentioned that UID: Unique identity number, unique at the national level for each citizen. So, there is no point of registration.

Answer 3

Answer is (A):

Option A: Note the given information about “BankAccount_Num”

BankAccount_Num: Unique account number at the bank. A student can have multiple accounts or joint accounts. This attribute stores the primary account number.

 

Note:- Here Multiple account or joint account means “if a student having Multiple account or joint account it means Two student can be joined together for joint Account(two student can have same account no), So we can not consider it as a Condidate key.”

           

Answer 4

COMMENT BELOW IF YOU HAVE ANY DOUBT

Comments

S ∩ UID = NULL what this means ?

Answer 5

Option (A) is Incorrect Because In multiple and Joint Bank Account  For Two or more person Bank account can be same....

That why Bank Account should not be a candidate Key...

Answer 6

One-line memory:
🔑 Candidate = Unique + Minimal; Superkey = Unique, extras allowed.