What is a foreign key used for in a database?

A foreign key is a critical element in relational databases that facilitates the establishment of relationships between different tables. It serves as a reference to the primary key of another table, thereby linking the two tables together. This relationship allows for data integrity and ensures that the data contained in the foreign key column corresponds to a valid entry in the associated table.

For example, if you have a 'Customers' table with a 'CustomerID' (which is the primary key), and an 'Orders' table that includes 'CustomerID' as a foreign key, this setup allows the database to maintain meaningful relationships. Each order can be associated with a specific customer, thus enabling efficient data retrieval and complex queries across related data sets.

In contrast, storing images and multimedia pertains to specific data types and not the relational aspect. Containing only the primary key would limit the functionality of a foreign key to merely referencing its own table rather than creating inter-table relationships. Summarizing entries of the same type does not relate to the concept of foreign keys; rather, it pertains to data aggregation tasks. Thus, using a foreign key primarily facilitates the creation and management of relationships between different tables, enabling a structured and normalized database design.