What is the role of a foreign key in relational databases?

A foreign key is a column or a set of columns in one table that references the primary key or a unique key in another table.

Its role is to ensure data integrity and establish relationships between tables:

• Referential integrity: Ensures that the values in the foreign key exist in the related primary or unique key of another table. This prevents "dangling" references.
• Defining relationships between tables: Shows how records in one table are related to records in another. This is the basis for JOIN operations in SQL.
• Action constraints: Allows setting rules for behavior when deleting or updating records in the parent table (e.g., ON DELETE CASCADE or ON UPDATE RESTRICT).

Example:

• The orders table contains a foreign key customer_id, which references the primary key id in the customers table.
• This guarantees that each order is associated with an existing customer.