带有两个表的 MySQL 子查询 count 和 group by

Question

I have database of doctors and hospital joined by hospitaldoctor table.

I have to List town, amount of hospitals in each town, but only hospitals which have more than 5 doctors.

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 

this is my query but MySQL returns me that subquery returns more than 1 row.

HOSPITAL

HOSPITAL DOCTOR

How i should write this query?

Answer 1

You can do what you want with basically the same structure:

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 ;

Note the following:

• You want to use in not = , because the subquery could return more than one row.
• The subquery should be returning the hospital id not the count.
• Use table aliases and qualified column names whenever a table refers to more than one table.

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