具有两个表计数和分组的MySQL子查询

Question

我有医生和医院的数据库，还有医院医生表。

我必须列出每个镇的医院数量，但只有有超过5名医生的医院。

SELECT hospital.town, count(town)
FROM hospital 
WHERE hospital.id = (
    SELECT count(hospital_id)
    FROM hospital_doctor GROUP BY hospital_id
    HAVING count(hospital_id)>5 )
GROUP BY town 

这是我的查询，但是MySQL会返回我的子查询返回超过1行。

医院

​

​

医院医生

​

​

我应该如何写这个查询？

Answer 1

你可以用基本相同的结构做你想做的事：

Select h.town, count(*)
from hospital h
where h.id in (select hd.hospital_id
               from hospital_doctor hd
               group by hd.hospital_id
               having count(*) > 5
              )
group by h.town ;

请注意以下事项：

• 您希望使用in而不是=，因为子查询可能返回多个行。
• 子查询应该返回医院标识，而不是计数。
• 每当表引用多个表时，请使用表别名和限定列名。

Answer 2

Select hospital.town, count(town) 
from hospital 
where 
    hospital.id in ( 
        select hospital_id 
        from hospital_doctor 
        group by hospital_id 
        having count(hospital_id)>5 
    ) 
group by town

Answer 3

SELECT h.town,  count(h.town)
FROM 
(
    SELECT hospital_id
    FROM hospital_doctor GROUP BY hospital_id
    HAVING count(doctor_id)>5 
) s1 
left outer join hospital h on (s1.hospital_id=h.id)  
GROUP BY h.town