A constraint can be disabled even if the constrained column contains data, which means the database does not enforce the constraint when it is disabled. A column with a UNIQUE constraint can contain a NULL value because NULLs are not considered equal to anything, including other NULLs, so multiple NULL values do not violate the UNIQUE constraint. A column with a FOREIGN KEY constraint can contain a NULL value, as it simply implies that the value must either be NULL or match a value in the referenced column.