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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?

  1. $\text {BankAccount_Num}$ is a candidate key
  2. $\text {Registration_Num}$ can be a primary key
  3. $\text {UID}$ is a candidate key if all students are from the same country
  4. If $S$ is a super key such that $S \cap \text{UID}$ is $\text{NULL}$ then $S \cup \text{UID}$ is also a superkey
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4 Answers

Best answer
71 votes
71 votes

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.

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17 votes
17 votes
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.
0 votes
0 votes

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.”

           

–2 votes
–2 votes
Option B

As a student can have two bank accounts so for this particular student the relation will contain two tuples but for these two tuples registration_num is same so it can't be a primary key..
Answer:

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