In relational database design, what constraint uniquely identifies each record in a table and cannot contain `NULL` values?
Answer and explanation
Correct answer: D. Primary Key
- Step 1: A Primary Key is a column (or combination of columns) that enforces entity integrity.
- Step 2: It must contain unique values for every row and strictly forbids null values (`NOT NULL`).
Answer
Primary Key.
Why not the others: 'Foreign Key' establishes referential links to primary keys in other tables; 'Default Constraint' provides fallback values; 'Check Constraint' validates business rules.
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