What is a foreign key and how does it enforce referential integrity?

• A. A foreign key is a column that references a primary (or unique) key in another table, enforcing that the referenced value exists.
• B. A foreign key is a primary key within the same table.
• C. A foreign key has no relation to other tables.
• D. A foreign key is automatically indexed and guarantees uniqueness.

Answer: (A)

A foreign key is a column (or set of columns) in one table that points to the primary key (or a unique key) in another table. This creates a link between the two tables and is how databases enforce referential integrity: every value in the foreign key must exist in the referenced table. If you try to insert or update a row with a value that doesn’t exist in the parent table, the database rejects it, preventing orphaned records and keeping relationships consistent. The foreign key can be NULL if the design allows, meaning no reference for that row. While foreign keys can be indexed for faster lookups, they are not guaranteed to be unique and they inherently involve a relationship across tables, unlike a key confined to a single table.