Consider the following two relations, named Customer and Person, in a database:
\[
\text{Person} \left(
\begin{array}{l}
\text{aadhaar CHAR(12) PRIMARY KEY,} \\
\text{name VARCHAR (32);}
\end{array}
\right)
\]
\[
\text{Customer} \left(
\begin{array}{l}
\text{name VARCHAR (32),} \\
\text{email VARCHAR(32) PRIMARY KEY,} \\
\text{phone CHAR(10),} \\
\text{aadhaar CHAR(12),} \\
\text{FOREIGN KEY (aadhaar) REFERENCES Person (aadhaar);}
\end{array}
\right)
\]
Which of the following statements is/are correct?
\[ \text{Person} \left( \begin{array}{l} \text{aadhaar CHAR(12) PRIMARY KEY,} \\ \text{name VARCHAR (32);} \end{array} \right) \]
\[ \text{Customer} \left( \begin{array}{l} \text{name VARCHAR (32),} \\ \text{email VARCHAR(32) PRIMARY KEY,} \\ \text{phone CHAR(10),} \\ \text{aadhaar CHAR(12),} \\ \text{FOREIGN KEY (aadhaar) REFERENCES Person (aadhaar);} \end{array} \right) \]
Which of the following statements is/are correct?
Show Hint
- aadhaar is a candidate key in the Customer relation
- phone can be NULL in the Customer relation
- aadhaar is a candidate key in the Person relation
- aadhaar can be NULL in the Person relation
The Correct Option is B, C
Solution and Explanation
Statement (A): Is aadhaar a candidate key in the Customer relation?
The aadhaar field in the Customer relation is a foreign key referencing the Person relation's aadhaar field, and it is not guaranteed to be unique in the Customer relation. Therefore, aadhaar cannot be a candidate key in Customer because a candidate key must be unique for each record in that table.
Thus, Option (A) is incorrect.
Statement (B): Can phone be NULL in the Customer relation?
In the Customer relation, the phone field is not specified as NOT NULL. Therefore, phone can be NULL in the Customer relation, which is allowed as per the given schema.
Thus, Option (B) is correct: phone can be NULL in the Customer relation.
Statement (C): Is aadhaar a candidate key in the Person relation?
In the Person relation, aadhaar is defined as the PRIMARY KEY, which by definition must be unique for each record. Therefore, aadhaar is a candidate key in the Person relation.
Thus, Option (C) is correct: aadhaar is a candidate key in the Person relation.
Statement (D): Can aadhaar be NULL in the Person relation?
The aadhaar field in the Person relation is defined as the PRIMARY KEY, and primary keys cannot be NULL. Therefore, aadhaar cannot be NULL in the Person relation.
Thus, Option (D) is incorrect: aadhaar cannot be NULL in the Person relation.
Conclusion:
The correct answers are Option (B) and Option (C).
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