14.1 Primary Key and Foreign Key in Plain English

Why. Company data is split across many tables so that each fact is stored once. Keys are how you find which rows in one table belong to rows in another.

What. A row is one record, such as one hub, and a column is one detail about it, such as its city. A primary key is a column whose value is different for every row, like hub_id in hubs. A foreign key is a column in another table that stores that value to point back, like hub_id in shipments. In SQLite, foreign key checks are usually off by default, but the way SQLite was built can change that. So run PRAGMA foreign_keys to see the current setting. If you need the checks, run PRAGMA foreign_keys = ON on each connection before you start a transaction, because the setting cannot be changed inside one.

How. Look at the hubs table, then at the hub_id column in shipments, and check the foreign key setting.

SELECT hub_id, hub_code, city, currency
FROM hubs
ORDER BY hub_id;

SELECT shipment_id, hub_id, tracking_code
FROM shipments
ORDER BY shipment_id;

PRAGMA foreign_keys;