What guarantees does a foreign key provide for data in a relational database?

A foreign key in a relational database guarantees data integrity between related tables. It ensures:

• Referential integrity: the foreign key values in the child table must match existing primary key values in the parent table or be NULL (if allowed).
• Prevention of deletion or modification of related data: the database will not allow deleting or modifying a record in the parent table if it is referenced by records in the child table, unless cascade delete or update is configured.

Thus, the foreign key guarantees that the relationships between tables remain correct and prevents the appearance of "orphan" links.