我如何选择每个医生有多少病人？

Question

我有一张医院就诊表，包括 DoctorID、PacientID、Date.... 我确实喜欢这个，但我收到了一个错误。

SELECT 
d.DoctorID, COUNT(v.DoctorID)
FROM Visits v
left join Doctors d on  v.DoctorID=d.DoctorID

Answer 1

你关心错过group by：

SELECT d.DoctorID, COUNT(v.DoctorID)
FROM Doctors d LEFT JOIN
     Visits v 
     ON v.DoctorID = d.DoctorID
GROUP BY d.DoctorID;

此外，您可能希望LEFT JOIN 以Doctors 开头，以便保留所有医生。

Answer 2

您的查询缺少GROUP BY 子句。此外，我认为不需要JOIN。如果您想要每个doctorID 的（不同）patientsIDs 计数，您可以直接从visit 表中获取该信息，如下所示：

select doctorID, count(distinct patientID) from visits group by doctorID

或者，如果您还想看没有病人的医生：

select d.doctorID, count(distinct v.patientID) 
from doctors d
left join visits v on v.doctorID = d.doctorID
group by d.doctorID

Answer 3

Live demo on db<>fiddle

您缺少GroupBy 子句。

所以你可以这样修复它

SELECT d.Name, COUNT(v.DoctorID) AS PantientCount
FROM Doctors d
LEFT JOIN Visits v ON d.DoctorId = v.DoctorId
GROUP BY d.Name

您也可以使用Conditional aggregation query with a group by，如下所示。

SELECT d.Name, SUM(CASE WHEN v.CustomerId IS NOT NULL THEN 1 ELSE 0 END) AS PantientCount
FROM Doctors d
LEFT JOIN Visits v ON d.DoctorId = v.DoctorId
GROUP BY d.Name

输出