Why should you care?

Bank transfer rows must reference real accounts — orphan ledger lines break balance reports and audits.

See it live — copy this example

Run in MySQL Workbench or the mysql CLI.

INSERT INTO orders (customer_id, order_ref, total_inr)
VALUES (999999, 'DF-BAD', 100.00);
-- Error 1452: foreign key constraint fails

What happened?

• customer_id 999999 does not exist in customers.
• InnoDB rejects the insert.
• Your app should catch this and show a friendly error.

Practice next

1. Confirm FK fk_orders_customer exists.
2. Attempt bad insert; read error code 1452.
3. Insert valid customer first; retry success.
4. SHOW CREATE TABLE orders\G and note CONSTRAINT clause.
5. Try UPDATE customer_id to invalid value — same failure.

Remember

FKs protect relationship validity. Failed inserts mean missing parent. Design PK types to match child FK exactly.

DataFlow checkout integrity

Stale session tries order for deleted customer — FK blocks ghost checkout.

Outcome: Revenue numbers always tie to real users.