Consider a relational table with a single record for each registered student…

2011

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

  1. Registration_Num: Unique registration number for 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 joint accounts. This attribute stores the primary account number.

  4. Name: Name of the student

  5. Hostel_Room: Room number of the hostel

Which of the following options is INCORRECT?

  1. A.

    BankAccount_Num is a candidate key

  2. B.

    Registration_Num can be a primary key

  3. C.

    UID is a candidate key if all students are from the same country

  4. D.

    If S is a super key such that S ∩ UID is NULL then S ∪ UID is also a superkey

Attempted by 602 students.

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Correct answer: A

Answer: BankAccount_Num is not a candidate key.

  • Reason BankAccount_Num fails: A candidate key must uniquely and minimally identify each record. Because accounts can be joint (shared by multiple students) and a student can have multiple accounts (or possibly no account), BankAccount_Num does not reliably provide uniqueness.

  • Why Registration_Num works: It is specified as unique for each registered student, so it can serve as a primary key.

  • Why UID can be a candidate key (given the condition): UID is unique at the national level, so if all students are from the same country, UID will uniquely identify each student.

  • Why adding UID to a superkey preserves the property: A superkey already uniquely identifies tuples; any superset of a superkey (such as adding UID) still uniquely identifies tuples, so the result is also a superkey.

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