What type of key refers to a key in one table that corresponds to a primary key in another table?

A foreign key is a key in one table that links to the primary key in another table. It establishes a relationship between the two tables in a relational database. When a foreign key is present, it ensures referential integrity, meaning that the data in the foreign key column must correspond to valid entries in the primary key column of the referenced table. This creates a connection between the records of the two tables, allowing data from different tables to be queried in relation to each other.

In databases, the foreign key is essential for creating relationships and enabling complex queries that combine data. For instance, if you have a "Customers" table with a primary key of CustomerID, and an "Orders" table that includes a CustomerID as a foreign key, you can easily retrieve all orders associated with a specific customer.

The other key types mentioned do not serve this specific relational purpose. A unique key ensures the uniqueness of values in a column but does not relate two tables. A primary key uniquely identifies records in its own table. An indexed key typically refers to a database structure that improves the speed of data retrieval but does not imply any relationship between tables.