SQL有两种外连接，分别是左连接和右连接
    1.外连接customers和orders两张表
    SELECT
    FROM customers c
    LEFT JOIN orders o
    ON c.customer_id = o.customer_id
    ORDER BY c.customer_id

    2.以product_id为主键，链接products表和order表
    SELECT *
    p.product_id,
    p.name,
    oi.quantity
    FROM products p
    LEFT JOIN order_items oi
    ON p.product_id = oi.product_id