Video narration
If an order stores customer ID 42, does that automatically mean customer 42 exists? Not necessarily. In an online store, the customers table holds accounts, and id is its primary key. The orders table stores customer_id. Customers is the parent; orders is the child. Matching numbers alone do not create enforcement. That is the job of a FOREIGN KEY: define it as FOREIGN KEY (customer_id) REFERENCES customers(id). It makes the database check that the referenced customer exists before accepting an order. An order for customer 42 is accepted; one for customer 999, with no matching row, is rejected. That is referential integrity: valid orders pass, broken references do not. Update and delete behavior can be configured, but the link is enforced. So, does storing an ID create the relationship? No. The FOREIGN KEY stops an order from pointing to a customer who isn't there.