Which key is a primary key from one table that is included in another table?

A foreign key is a primary key from one table that is referenced within another table. It establishes a relationship between the two tables, allowing for data integrity and enabling operations like joins to occur between the tables. By including a primary key from one table as a foreign key in another, the database can enforce constraints that ensure that relationships between the data are consistent. For instance, if you have a table for customers with a primary key of customer_id and another table for orders, the orders table would include a customer_id as a foreign key, linking each order to the customer who placed it. This relationship can be critical for database queries that need to access related information across multiple tables.

In contrast, other options like a primary key refer to the unique identifier within a single table rather than a linking mechanism between tables. A composite key involves a combination of two or more columns to uniquely identify rows in a table but does not specifically refer to relationships. An index key is utilized to enhance the speed of data retrieval but does not inherently define relational links between tables.