Which statement best describes a unique constraint in terms of nullability?

Which statement best describes a unique constraint in terms of nullability?

• A. It enforces uniqueness and can allow null values depending on the database.
• B. It disallows all null values in the column.
• C. It guarantees the column is a primary key.
• D. It always prevents null values.

Answer: (A)

A unique constraint makes sure the values in the constrained column (or set of columns) are all different. How it treats nulls is separate: NULL is considered an unknown value, and in practice many databases don’t count NULLs as duplicates. So a column with a unique constraint can still allow NULLs, but exactly how NULLs are handled can vary by database. If you want to forbid NULLs entirely, you’d add NOT NULL (or use a primary key, which implies NOT NULL).

That’s why this option is the best: it captures both the uniqueness requirement and the variable behavior of NULLs across systems. The other statements misstate the effect of a unique constraint on nulls: it doesn’t automatically disallow NULLs in all systems, it doesn’t make the column a primary key, and it doesn’t always prevent NULL values.