Question

我有两个表，一个包含有关患者访问的信息：PatientName，DoctorName，DataOfVisit等，第二个表包含有关医生的信息：DoctorName和DoctorSpeciality。

我需要创建一个查询，向我打印PatientName，患者去过的医生数量以及不同专业的数量。

如果我跑

SELECT PatientName, COUNT(VISITS.DoctorName) as DocNum,   Count(DoctorSpeciality) as SpecNum
FROM VISITS 
INNER JOIN Doctors 
ON VISITS.DoctorName = Doctors.DoctorName
GROUP BY PatientName, VISITS.DoctorName, DoctorSpeciality

我得到了医生的数量，但没有得到专业的数量，患者没有分组。

Answer 1

在大多数数据库中，您只需使用count(distinct specialty)。但是，Access并不支持这一点。

您可以使用两个group by来执行您想要的操作：

SELECT PatientName, COUNT(*) as NumSpecialties, SUM(NumDocs) as NumDocs
FROM (SELECT PatientName, Doctors.DoctorSpeciality, COUNT(*) as NumDocs 
      FROM VISITS INNER JOIN
           Doctors 
            ON VISITS.DoctorName = Doctors.DoctorName
      GROUP BY PatientName, DoctorSpeciality
     ) as pd
GROUP BY PatientName;

Answer 2

您可以在COUNT（）函数中添加DISTINCT，也不应在GROUP BY中包含聚合字段。

以下是我将如何更改查询。

SELECT PatientName, COUNT(DISTINCT VISITS.DoctorName) as DocNum,       
Count(DISTINCT DoctorSpeciality) as SpecNum
FROM VISITS INNER JOIN
Doctors ON VISITS.DoctorName = Doctors.DoctorName
GROUP BY PatientName

Answer 3

将表名用于要从db

中选择的字段

SELECT VISITS.PatientName，COUNT（VISITS.DoctorName）为DocNum，Count（Doctors.DoctorSpeciality）为SpecNum 来自参观 INNER JOIN医生 ON VISITS.DoctorName = Doctors.DoctorName GROUP BY PatientName，VISITS.DoctorName，DoctorSpeciality

我如何计算访问sql中的2个不同的表

3 个答案: