What is a foreign key?

A foreign key is a crucial component in relational database management that establishes a link between two tables. Specifically, it is an attribute (or a set of attributes) in one table that refers to the primary key of another table. This relationship is essential for maintaining referential integrity within the database.

By utilizing a foreign key, the database ensures that the data remains consistent across related tables, enforcing constraints that prevent actions that would leave orphaned records—records that reference a non-existent primary key. For example, consider two tables: "Customers" and "Orders." The "Orders" table may have a foreign key that references the "CustomerID" primary key in the "Customers" table. This setup allows the database to connect orders to the specific customers who placed them, providing a coherent structure to the data.

The concept of a foreign key is fundamental in establishing relationships between tables, enhancing the database's ability to handle complex queries and maintain data integrity.