Consider a relational table with a single record for each registered student with the following attributes.
1. Registration_Num: Unique registration number of each registered student 2. UID: Unique identity number, unique at the national level for each citizen 3. BankAccount_Num: Unique account number at the bank. A student can have multiple accounts or join accounts. This attribute stores the primary account number. 4. Name: Name of the student 5. Hostel_Room: Room number of the hostel
Which one of the following option is INCORRECT?
(A) BankAccount_Num is candidate key
(B) Registration_Num can be a primary key
(C) UID is candidate key if all students are from the same country
(D) If S is a superkey such that S∩UID is NULL then S∪UID is also a superkey
Answer: (A)
Explanation: A Candidate Key value must uniquely identify the corresponding row in table. BankAccount_Number is not a candidate key. As per the question “A student can have multiple accounts or joint accounts. This attributes stores the primary account number”. If two students have a joint account and if the joint account is their primary account, then BankAccount_Number value cannot uniquely identify a row.
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