MySql的分組和過濾結果：錯誤

Question

我需要幫助： 此查詢工作得很好

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; 

但是當我想篩選

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; 

我得到這個錯誤

錯誤1064（42000）：您的SQL語法有錯誤;檢查對應於您的MySQL服務器版本的手冊，以便在第6行'where nb> 0'附近使用正確的語法。

我的查詢有什麼問題？

Answer 1

條件的分組功能應放入HAVING子句：

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; 

Answer 2

排序應該永遠是最後

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