1.外连接customers/orders/shippers三张表
    SELECT
    c.customer_id,
    c.first_name,
    o.order_id,
    sh.shipper
    FROM customers c
    LEFT JOIN orders o
    ON c.customer_id = o.customer_id
    LEFT JOIN shippers sh
    ON o.shipper_id = sh.shipper_id
    ORDER BY c.customer_id

    2.外连接orders/customers/shippers/order_statuses四张表
    SELECT
    o.order_id,
    o.order_date,
    c.first_name AS customer
    sh.name AS shipper
    os.name AS status
    FROM orders o
    JOIN customers c
    ON o.customer_id = c.customer_id
    LEFT JOIN shippers sh
    ON o.shipper_id = sh.shipper_id
    JOIN order_statuses os
    ON o.status = os.order_status_id