Which of the following constraints prevents use of null values in a database?

The NOT NULL constraint is specifically designed to prevent null values from being inserted into a database column. When this constraint is applied to a column, it mandates that every record must have a value for that column; it cannot be left empty or set to null. This is particularly useful in maintaining data integrity, as it ensures essential fields, such as usernames or email addresses, always contain valid data, thereby avoiding ambiguous records.

The other constraints serve different purposes: while the PRIMARY KEY constraint also disallows null values since it uniquely identifies each record in a table, its primary role is to ensure that the values in that column or set of columns are unique. Similarly, a FOREIGN KEY establishes a relationship between tables but does not inherently prevent null values in the child table unless combined with a NOT NULL constraint. The UNIQUE constraint allows for null values unless specifically specified with NOT NULL, ensuring all non-null entries of a column are distinct but permitting the presence of NULLs with no restrictions. This makes NOT NULL the clear choice for preventing null values on its own.