What role does a foreign key serve in a relational database?

A foreign key serves a crucial purpose in relational databases by establishing a relationship between records in two different tables. This relationship allows for referential integrity, meaning that the foreign key in one table points to a primary key in another table. By doing so, it ensures that the data remains consistent across those tables. For instance, if you have a "Customers" table and an "Orders" table, the "Orders" table may include a foreign key that references the customer ID from the "Customers" table. This relationship helps maintain data integrity by linking orders to the customers who made them, allowing for efficient data retrieval and analysis.

The other options describe characteristics that do not accurately define the role of a foreign key. A foreign key does not uniquely identify rows in its own table; that role is reserved for primary keys. Additionally, while a foreign key can be a primary key in another table, it is not always the case that it must be one. Lastly, foreign keys can accept null values, representing records in the corresponding table that do not have an associated entry. Therefore, the role of a foreign key as a connector between tables is an essential aspect of designing and utilizing relational databases effectively.