一道SQL题目，效率提高一个数量级

有一张表order(orderid,userid,amount)，写一条查询语句，取出所有消费总额超过100的用户的所有订单号orderid。

#本人开始是这样想的
SELECT aa.userid,aa.orderid,aa.amount FROM `order` aa  LEFT JOIN 
(
SELECT SUM(amount) AS amount,userid FROM `order` GROUP BY `userid`
)  tt ON tt.userid = aa.userid AND tt.amount > 100

#看了高人的写法，效率提高一个数量级，表只需要遍历一次

SELECT userid ,GROUP_CONCAT(`orderid`)AS order_id,SUM(amount) AS amount
FROM order 
WHERE amount > 100
GROUP BY userid