What does the NOT NULL constraint ensure for a column?

• A. The column must contain unique values
• B. The column cannot contain NULL values
• C. The column must have a default value
• D. The column must only contain numeric values

Answer: (B)

The NOT NULL constraint is used in a database management system to enforce that a particular column cannot contain NULL values. This means that every row in the table must have a valid entry for that column; it cannot be left empty or undefined. This constraint is crucial in scenarios where a certain piece of data is essential for the integrity of the record, such as a user ID in a login table or an order date in an order processing table.

While protecting against NULL values is necessary to maintain data integrity, other constraints or features in databases serve different purposes. For instance, ensuring unique values in a column relates to the UNIQUE constraint, which is separate from the NOT NULL constraint. Similarly, having a default value pertains to default settings for when no value is supplied, and restrictions on data types, like numeric values, are enforced through data type definitions rather than the NOT NULL constraint. Each of these functionalities serves distinct roles in managing and ensuring data quality within a database system.