先说一下SQL执行顺序：
from => join => on => where => group by => having => select => order by => limit
having、where：都是条件查询，区别在于having可以用字段别名和聚合函数。
具体区别见：https://blog.csdn.net/W_Kenneth/article/details/110950145

测试数据表：reward

要求：查询用户最新的奖金数：
结果：

正确写法：

1、SELECT r.uid,r.money FROM (
SELECT MAX(id) id FROM reward GROUP BY uid
) a LEFT JOIN reward r ON a.id = r.id;

2、SELECT uid, money FROM (
SELECT * FROM reward ORDER BY id desc limit 4
) as a GROUP BY a.uid;
– limit需要大于最后的结果的条数
– mysql5.7之后，子查询中的排序不生效，加上limit使排序生效

错误写法：

SELECT id,uid,money FROM reward ORDER BY id desc GROUP BY uid;
直接报错：