加入和分组问题

Question

我的医院数据库中有四张桌子。我需要显示所有患者的姓名，并为每位患者显示他们不止一次访问过的医生的姓名，以及每位医生的访问次数。如果患者没有去过任何医生，或者没有多次访问任何医生，管理层不希望在列表中看到他们。编写一个查询，以便为管理层检索此信息。

 Select * from Medical_History;
 Select * from Physician;
 Select * from Appointment;
 Select * from Patient;

我已经进行了连接查询以及组查询，但我不知道如何组合它们。因为我正在分组错误。因为我需要显示患者到医生的就诊次数，并且必须不止一次。所以需要帮助。这是加入和组子句：

  Select Patient_First_Name,    Patient_Last_Name,Physician_First_Name,Physician_Last_Name
         from Patient Join Medical_History on   Patient.Patient_ID=Medical_History.Patient_ID
        Join Physician on Medical_History.Physician_ID=Physician.Physician_ID;

        select Physician_ID, count(Patient_ID)
        from Medical_History
        Group by Physician_ID, Patient_ID;

Answer 1

您可以先将GROUP BY与HAVING一起使用，以获取患者和医生的关联，然后加入每个主表以获取其姓名。

这样的事情：

SELECT 
  Patient_First_Name,
  Patient_Last_Name,
  Physician_First_Name,
  Physician_Last_Name,
  TotalVisits
FROM Patient P
INNER JOIN 
(
    SELECT Patient_ID, Physician_ID, COUNT(*) as TotalVisits
    FROM Medical_History
    GROUP BY Physician_ID, Patient_ID
    HAVING COUNT(*) >= 2
) PP ON P.Patient_ID = PP.Patient_ID
INNER JOIN Physician PH ON PH.Physician_ID = PP.Physician_ID;

Answer 2

我会CROSS APPLY计算每个Patient记录，如下所示：

SELECT Patient_First_Name, Patient_Last_Name,Physician_First_Name,Physician_Last_Name
FROM Patient
CROSS APPLY (SELECT Physician_First_Name,Physician_Last_Name,COUNT(*) TotalVisits
             FROM Medical_History
             JOIN Physician on Medical_History.Physician_ID=Physician.Physician_ID
             WHERE Medical_History.Patient_ID=Patient.Patient_ID
             GROUP BY Physician_First_Name,Physician_Last_Name
             HAVING COUNT(*)>1) T

内部查询选择与Patient具有多个关联的记录。