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

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..

Comments

Der is single tuple bcoz Bank_Account_no stores information regarding primary bank account no only.