sql里有两种连接方式，分别为内连接和外连接

    1.根据customer_id主键， 合并orders表和customers表里的列
    SELECT *
    FROM orders
    INNER JOIN customers
    ON orders.customer_id = customers.customer_id

    默认内连接：
    SELECT
    FROM orders
    JOIN customers
    *ON     orders.customer_id = customers.customer_id

    2.从合并表中选出订单id, 消费者id, first_name, last_name
    这里的customer_id需要明确是哪张表的消费者id,否则是
    不明确的。（虽然我也不知道为啥不明确）
    SELECT order_id, orders.customer_id, first_name, last_name
    FROM orders
    JOIN customers
    ON orders.customer_id = customers.customer_id

    3.从合并表中选出订单id, 消费者id, first_name, last_name
    这里的customer_id需要明确是哪张表的消费者id,否则是
    不明确的。增加表的简写。
    SELECT order_id, o.customer_id, first_name, last_name
    FROM orders o
    JOIN customers c
    ON o.customer_id = c.customer_id

    4.连接order_items和products两张表
    SELECT *
    FROM order_items oi
    JOIN products p
    ON oi.product_id = p.product_id