What is the purpose of a foreign key?

The purpose of a foreign key in a relational database is to create a link between two tables. A foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table. This establishes a relationship between the two tables, allowing you to join them in queries and enforce referential integrity.

For example, in a database that includes a table for customers and another for orders, the orders table might have a foreign key that corresponds to the customer ID in the customers table. This relationship ensures that every order is linked to a specific customer, thereby maintaining the integrity of the data and allowing for complex data retrieval operations.

The other choices focus on different functionalities not related to the role of foreign keys. While encryption is important for protecting sensitive information, indexing helps speed up data retrieval, and storing backup copies pertains to data safety and redundancy, none of these involve the direct linking of tables like foreign keys do.