Foreign key
Also called: FK
A column that points to the primary key of a row in another table, like orders.user_id → users.id.
Definition
Foreign keys link tables together and are what you join on. If the database doesn't enforce them, you can get orphan rows: records that point to something that no longer exists. Testers look for these after deletes and migrations.
In practice
An order item with order_id = 1016 when there is no order 1016 is an orphan row, usually caused by deleting an order but not its items.
Learn it in context
Lessons that use this term.