1.忘记写from

2.忘记分组：求每个人的积分总和要先把每个人先分组

3.有group by的情况下，select输出的只能是聚合函数/在group by 被分组的

4.order by,group by 用”,”逗号连接，不能用and连接

5.date_format后面没有e

6.日期类型经过 DATE_FORMAT()后变成 字符串，所以使用right()函数取后两位即为月数

7.group by可以用窗口函数代替

8.求最早两次下单的日期

善用max(date) min(date)

select tmp2.user_id, min(tmp2.date), max(tmp2.date), tmp2.cnt
from 
    (select tmp1.user_id, tmp1.date, tmp1.cnt
    from 
        (select user_id, date, rank() over(partition by user_id order by date) rankNo, 
        count(*) over(partition by user_id) cnt
        from order_info
        where date>'2025-10-15' and order_info.status = 'completed' 
        and order_info.product_name in('C++','Java','Python')) tmp1
    where tmp1.cnt>1 and rankNo<=2) tmp2
group by tmp2.user_id
order by tmp2.user_id;