MySql组和筛选结果：错误

我需要帮助： 这个查询非常有效

select count(distinct(a.mat)),count(distinct(c.mail)) as nb 
from aca as a 
left join c on c.mat=a.mat 
group by a.mat
order by nb desc;

但是当我想过滤的时候

select count(distinct(a.mat)),count(distinct(c.mail)) as nb 
from aca as a 
left join c on c.mat=a.mat 
group by a.mat
order by nb desc
where nb > 0;

我得到这个错误

错误1064（42000）：您的SQL语法有错误；查看与MySQL服务器版本对应的手册，以了解第6行“where nb>0”附近要使用的正确语法

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我的查询出了什么问题？

Order by应该始终是最后一个

select count(distinct(a.mat)),count(distinct(c.mail)) as nb 
from aca as a 
left join c on c.mat=a.mat 
group by a.mat
having nb > 0
order by nb desc

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分组函数的条件应放在HAVING子句中：

select count(distinct(a.mat)),count(distinct(c.mail)) as nb 
from aca as a 
left join c on c.mat=a.mat 
group by a.mat
having nb > 0
order by nb desc;

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忘了它的总量！编辑