In a relational database, what is the purpose of a foreign key?

A foreign key is a column (or a set of columns) in one table that refers to the primary key in another table. This establishes a relationship between the two tables, enforcing referential integrity. When a foreign key is created, it ensures that any value in that column must either be null or match an existing value in the primary key column of the referenced table. This linkage allows for more complex queries and relationships, such as one-to-many or many-to-many associations, enhancing the overall data structure of a relational database. Option A is incorrect because uniquely identifying a record is the role of the primary key, not the foreign key. Option C is incorrect since ensuring data integrity within a single table relates more to constraints such as primary keys and unique constraints. Option D is incorrect because while foreign keys can affect query performance, their primary purpose is not performance enhancement but establishing relationships. Option E is incorrect as foreign keys do not specifically deal with the storage of large text data, which is typically managed through data types like TEXT or BLOB.