What does a foreign key do?

• A. Enforces a unique constraint within the same table
• B. Encrypts data across tables
• C. References a primary key in another table and maintains referential integrity
• D. Automatically creates a new table

Answer: (C)

A foreign key establishes a link between two tables by using a column (or set of columns) in one table that points to the primary key in another table. This creates a defined relationship and enforces referential integrity, meaning every value in the foreign key must match an existing primary key value in the referenced table. In practical terms, it prevents references to non-existent rows and makes it possible to combine related data from both tables with joins. Depending on the schema, the foreign key can be optional (allowing NULL) or mandatory, and actions like cascade or restrict can control what happens when the referenced row is updated or deleted.