What does a foreign key achieve in a relational database?

A foreign key plays a vital role in maintaining relationships between tables in a relational database. Specifically, it links two tables by referencing a primary key from another table. This linkage establishes a connection between records in different tables, thus allowing for the enforcement of referential integrity.

When a foreign key is defined in one table, it ensures that the values in this column correspond to valid entries in the primary key column of another table. For example, in a database with a 'Students' table and a 'Courses' table, a foreign key in the 'Courses' table can reference the primary key of the 'Students' table, creating a relationship that allows for queries and data retrieval that reflect these connections.

This relationship is crucial for operations such as joining tables, which allow for complex queries where data from multiple tables can be aggregated and analyzed efficiently. Such a mechanism also prevents orphaned records, ensuring that every entry in the referencing table corresponds to a valid entry in the referenced table.