IFNULL() return 列值为 int 的字符串可以使用 MySQL 吗？

Question

我的查询

select cname, count(screening_occapancy.idclient) as 'Count'
from client, screening_occapancy 
where client.client_no = screening_occapancy.idclient 
group by cname

returns 以下:

Name        Count
Name1        2
Name2        3
Name3        6

etc，现在我希望它在 'Count' 中的值是 "not found" 如果值为 null 或 0，这可能吗？我的结果中需要这样的东西：

    Name        Count
    Name1         2
    Name2         3
    Name3    "Not found"

Answer 1

使用 left join 为所有未找到的匹配项获取 0

select c.cname, 
       count(so.idclient) as 'Count'
from client c
left join screening_occapancy so on c.client_no = so.idclient 
group by c.cname

顺便说一句，不再使用遗留的隐式连接语法。使用显式连接。

Answer 2

    Select cname , 
case when Count is null or count =0 then 'Not found' 
else Count end as count
 from
    (select cname,
 count(screening_occapancy.idclient) as 'Count' 
    from client left join screening_occapancy 
    on
    client.client_no = screening_occapancy.idclient group by cname) t

在您的查询上方写一个包装器查询以检查计数列

Answer 3

select cname, IF(count(screening_occapancy.idclient)!=0,count(screening_occapancy.idclient),'NOT FOUND') as 'Count'
from client, screening_occapancy 
where client.client_no = screening_occapancy.idclient 
group by cname

或者如果计数 returns 为空？

select cname, IFNULL(count(screening_occapancy.idclient),'NOT FOUND') as 'Count'
from client, screening_occapancy 
where client.client_no = screening_occapancy.idclient 
group by cname

IFNULL() return 列值为 int 的字符串可以使用 MySQL 吗？

Can IFNULL() return a string where the column value is int, using MySQL?

mysql

ifnull