Which statement about the UNIQUE constraint is correct?

• A. It ensures a column has NULL values
• B. It restricts columns to duplicate values
• C. It can be applied at both column and table levels
• D. It automatically generates values for the column

Answer: (C)

The UNIQUE constraint is a database feature that ensures all values in a column or a set of columns are distinct from one another, thereby preventing duplicate entries. When considering the options provided, the statement that it can be applied at both column and table levels is accurate.

Applying the UNIQUE constraint at the column level means that each value in that specific column must be unique across all rows in the table. This is typically used for columns like email addresses or usernames, where duplicates must not occur. When applied at the table level, it allows for a combination of columns to be unique as a whole, rather than just on a single column basis.

Just to clarify some surrounding concepts: the UNIQUE constraint does not ensure a column has NULL values, as NULL is considered distinct and can be allowed unless specified otherwise. It must also be noted that the UNIQUE constraint does not generate values for the column; that aspect is typically associated with the AUTO_INCREMENT feature used for primary keys. Therefore, the flexibility of applying the UNIQUE constraint both at the column level and across multiple columns at the table level makes this option the correct choice.