Sql server sql命令，用于展平具有多对多关系的2个表

我有两个具有多对多关系的表。考虑下面的用例

患者

id | Name
---+------------------
1    jack
2    sara
3    patrick

id | Disease
---+-----------------
1    Diabetes
2    Cancer
3    Epilepsy

疾病

id | Name
---+------------------
1    jack
2    sara
3    patrick

id | Disease
---+-----------------
1    Diabetes
2    Cancer
3    Epilepsy

每个患者都可能患有多种疾病。连接表为：

耐心等待

Patient_id | Disease_id
-----------+--------------
1                1
1                3
2                2

我想列出每个病人的疾病

尝试

id | Name
---+------------------
1    jack
2    sara
3    patrick

id | Disease
---+-----------------
1    Diabetes
2    Cancer
3    Epilepsy

以下查询为患有一种或多种疾病的患者创建半正确的结果：

SELECT 
    Patients.Name,  
    Diseases = STUFF((SELECT ',' + dbo.Diseases.Name 
                      FROM dbo.Diseases 
                      FOR  XML PATH(''), type).value('.', 'NVARCHAR(MAX)'), 1, 1, '')
FROM dbo.Patients
LEFT JOIN 
    dbo.PatientDiseases ON PatientDiseases.Patient_Id = Patients.Id
JOIN 
    dbo.Diseases ON Diseases.Id = PatientDiseases.Disease_Id

如何编写一个查询来生成这样的输出

name    | diseases
--------+-------------------
jack    | diabetes, Epilepsy
sara    | cancer
patrick | NULL 

---

看起来你应该做的是：

SELECT P.[Name],
       STUFF((SELECT ',' + D.Disease
              FROM dbo.PatientDisease PD
                   JOIN dbo.Diseases D ON PD.Disease_id = D.id
              WHERE PD.Patient_id = P.id
              FOR XML PATH(''),TYPE).value('.','nvarchar(MAX)'),1,1,'') AS Diseases
FROM dbo.Patients P;

如果您使用的是SQL Server 2017，则可以使用

STRING\u AGG

：

SELECT P.[Name],
       STRING_AGG(D.Disease,',') WITHIN GROUP (ORDER BY D.Disease) AS Diseases
FROM dbo.Patients P
    LEFT JOIN dbo.PatientDisease PD ON P.id = PD.Patient_id
    LEFT JOIN dbo.Diseases D ON PD.Disease_id = D.id
GROUP BY P.[Name];

---

您提供的查询无法运行，它没有来自的

。您是对的。修好它