How a foreign key works: why the database refuses customer 9

How a foreign key works: why the database refuses customer 9

Order 104 points at customer 9. There is no customer 9, so the database refuses it.

A foreign key is a column that has to match a real row in another table. It also stops you deleting a customer who still has orders, unless you ask for cascade, which takes her orders with her.

Save it for the next time you see FOREIGN KEY constraint failed.

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