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    Question

    In a relational database, which key uniquely identifies

    tuples within a table and can never contain NULL values?
    A Primary Key Correct Answer Incorrect Answer
    B Foreign Key Correct Answer Incorrect Answer
    C Candidate Key Correct Answer Incorrect Answer
    D Alternate Key Correct Answer Incorrect Answer
    E Composite Key Correct Answer Incorrect Answer

    Solution

    A Primary Key uniquely identifies each record (or tuple) in a table and cannot contain NULL values. It enforces entity integrity, ensuring that every row in a table has a unique identifier. For example, in a table of employees, an employee ID serves as a primary key, ensuring no duplicate or NULL entries exist. Why Other Options Are Incorrect:

    • Foreign Key: Links one table to another and can contain NULL values in some cases; its purpose is to enforce referential integrity, not uniqueness within its table.
    • Candidate Key: A potential key that can uniquely identify tuples but is not chosen as the primary key.
    • Alternate Key: A candidate key not selected as the primary key but still capable of uniquely identifying records.
    • Composite Key: A primary key consisting of multiple columns but is a type of primary key, not a separate concept.

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