In relational databases, what is the role of a foreign key?

• A. It uniquely identifies a record in its own table
• B. It links a row in one table to a row in another table
• C. It stores metadata about the database
• D. It is a substitute for a primary key in the same table

Answer: (B)

The main idea is that a foreign key creates connections between tables by storing the value of a row’s key from another table. This links a row in one table to a row in another table, enabling relationships such as which customer an order belongs to. This linkage allows you to combine related data across tables and keeps data consistent, because you can’t reference a non-existent row in the linked table.

Think of a customers table and an orders table: each order might store a customer_id that matches a customer’s primary key in the customers table. That customer_id is the foreign key. It doesn’t have to be unique within the orders table, and it may be null if the relationship is optional, but it ensures the referenced customer exists.

In contrast, a primary key uniquely identifies a row within its own table, which is not the role of a foreign key. A foreign key does not store metadata about the database, and it isn’t a substitute for a primary key in the same table.