Which of the following is a characteristic of NOT NULL constraint in RDBMS?

The characteristic of a NOT NULL constraint in a relational database management system (RDBMS) is that it prevents any entries in a column from being null. When a NOT NULL constraint is applied to a column, it ensures that every row must contain a valid value for that column. This is crucial for maintaining data integrity, as there are scenarios where null values would not make sense in the context of the database. For instance, if you had a table to store user information, having a NOT NULL constraint on the "email" column would ensure that every user has a registered email address, preventing the possibility of having incomplete data.

In contrast to this, allowing duplicate entries relates to a different aspect of database design and is not influenced by the NOT NULL constraint. While NOT NULL ensures that a column must have a value, it does not enforce uniqueness among the entries. Unique value enforcement is handled by a unique constraint. Similarly, the ONLY requirement for foreign keys is to ensure referential integrity between tables, which does not directly relate to the NOT NULL constraint. Therefore, the ability of NOT NULL to mandate specific data presence is its defining characteristic.