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 ensures that the values in a column (or a set of columns) are distinct. How it treats nulls isn’t fixed across all systems: in many databases, NULL is not considered equal to anything, including another NULL, so you can have multiple rows with NULL in that column while still enforcing uniqueness for all non-null values. If you truly want to forbid NULLs, you’d add a NOT NULL constraint (or use a primary key, which implies NOT NULL). This is why the statement that best describes it is that a unique constraint enforces uniqueness and can allow null values depending on the database.

A unique constraint ensures that the values in a column (or a set of columns) are distinct. How it treats nulls isn’t fixed across all systems: in many databases, NULL is not considered equal to anything, including another NULL, so you can have multiple rows with NULL in that column while still enforcing uniqueness for all non-null values. If you truly want to forbid NULLs, you’d add a NOT NULL constraint (or use a primary key, which implies NOT NULL). This is why the statement that best describes it is that a unique constraint enforces uniqueness and can allow null values depending on the database.