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

返回以下内容：

Name        Count
Name1        2
Name2        3
Name3        6

等，现在我想要它的价值＆＃39; Count＆＃39;未被发现＆＃34;如果值为null或0，那可能吗？我在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获取所有未找到匹配项的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

BTW不再使用遗留隐式连接语法。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count返回null？

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（）可以使用MySQL返回列值为int的字符串吗？

3 个答案: