A student table has one record per student: Registration_Num unique per student; UID unique nationally per citizen; BankAccount_Num is the primary bank account (a student may have several or joint accounts); Name; Hostel_Room. Which is incorrect?
BankAccount_Num is a candidate key
Registration_Num can be a primary key
UID is a candidate key if all students are from the same country
Correct answer: (A) BankAccount_Num is a candidate key
Explanation
A joint bank account can be shared by students, so primary account number need not uniquely identify a student and is not guaranteed a candidate key, A. Registration is unique; nationally unique UID works under the same-country premise; adding an attribute to a superkey preserves uniqueness.
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