用0将mysql查询中的值替换为null

Question

I am trying replace null with 0 with following statement but returns no recrods instead of of catid supplied and 0.

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

Answer 1

As far as I know, COUNT(*) does never return NULL. It returns 0 if there is no record.

Answer 2

count(*) never returns NULL , so you don't need any conditional logic:

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

Perhaps your issue is that the query is returning no rows . If so, you can leave out the group by . Then the query will always return one row:

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