用0替换mysql查询中的null值

Question

我正在尝试使用以下语句将0替换为0但不返回recrods而不是catid和0。

select ifnull(count(*),0) as days, catid from mytable where Id=48 and catId=7 
group by mytable.catId;

Answer 1

据我所知，COUNT（*）永远不会返回NULL。如果没有记录，则返回0.

Answer 2

count(*)永远不会返回NULL，因此您不需要任何条件逻辑：

select count(*) as days, catid
from mytable
where Id = 48 and catId = 7 
group by mytable.catId;

也许您的问题是查询没有返回行。如果是这样，您可以省略group by。然后查询将始终返回一行：

select count(*) as days, catid
from mytable
where Id = 48 and catId = 7 ;