1.对于每一个客户中的某一个客户，
    连接所有的产品品类。
    SELECT
    c.first_name AS customer,
    p.name AS product
    FROM customers c
    CROSS JOIN products p
    ORDER BY c.first_name

    隐式写法：
    SELECT
    c.first_name AS customer,
    p.name AS product
    FROM customers c, products p