Which integrity rule states that no attribute of a primary key may contain NULL?
Correct answer C. Entity integrity
Explanation
Entity integrity requires every primary key value to be present and unique, because the key is how each tuple (entity) is identified; a NULL would mean an unidentifiable row. SQL enforces this by making PRIMARY KEY imply NOT NULL and UNIQUE. Referential integrity is the separate rule that a foreign key value must match an existing primary key value or be NULL.
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