INNER JOIN

-- suppliers + products
SELECT product_name, suppliers.company_name, units_in_stock
FROM products
INNER JOIN suppliers ON products.supplier_id = suppliers.supplier_id
ORDER BY units_in_stock DESC

-- categories + products
SELECT category_name, SUM(units_in_stock)
FROM products
INNER JOIN categories ON products.category_id = categories.category_id
GROUP BY category_name
ORDER BY SUM(units_in_stock)

-- more than 1 INNER JOIN
SELECT order_date, product_name, ship_country, products.unit_price, quantity, discount
FROM orders
INNER JOIN order_details ON orders.order_id = order_details.order_id
INNER JOIN products ON order_details.product_id = products.product_id

Можно сокращенно писать JOIN . Это тоже самое что INNER JOIN .

--  Найти заказчиков и обслуживающих их заказы сотрудников 
-- таких, что и заказчики и сотрудники из города London, 
-- а доставка идёт компанией Speedy Express. 
-- Вывести компанию заказчика и ФИО сотрудника.
SELECT c.company_name, CONCAT(e.first_name, ' ', e.last_name)
FROM orders AS o
JOIN customers AS c USING(customer_id)
JOIN employees AS e USING(employee_id)
JOIN shippers AS s ON o.ship_via = s.shipper_id
WHERE c.city = 'London' AND e.city = 'London' AND s.company_name = 'Speedy Express';