The true statement regarding a foreign key in a relational database is:
Null values are allowed.
Foreign keys can accept null values unless explicitly defined otherwise. This means that a record can exist in a table without a corresponding record in the referenced table. Here is a brief clarification of the other options:
- They are required. - This is false because foreign keys can be nullable.
- They are automatically indexed. - This is also false; foreign keys are not automatically indexed, though indexing them can improve performance.
- There can be only one per table. - This statement is false; a table can have multiple foreign keys referencing different tables or even the same table in different relationships.