GATE CS 2011 Q.12 (CS Q.12): ER Model and Relational Schema Design

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

Registration_Num: Unique registration number of each registered student
UID: Unique identity number, unique at the national level for each citizen
BankAccount_Num: Unique account number at the bank. A student can have multiple accounts or joint accounts. This attribute stores the primary account number.
Name: Name of the student
Hostel_Room: Room number of the hostel

Which of the following options 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

If $S$ is a superkey such that $S \setminus \{UID\}$ is NULL then $S \cup \{UID\}$ is also a superkey

Official answer and solution

Solution

Analyze candidate key requirements and uniqueness properties on each attribute of the relational table.

A candidate key is a minimal superkey that must uniquely identify every tuple in the relation.

Statement A: The problem explicitly states that students can have joint bank accounts. When two students share a joint account, multiple student records can possess the exact same primary BankAccount_Num. Because uniqueness is violated, BankAccount_Num cannot be a candidate key. Statement A is INCORRECT.

Statement B: Registration_Num is unique for each registered student, so it qualifies as a candidate key and can serve as the primary key.

Statement C: If all students belong to the same country, their national UID is unique across all individuals, making it a valid candidate key.

Statement D: Any superset of a superkey is also a superkey, so adding UID to an existing superkey $S$ preserves superkey status.

Final answer

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