SQL中动态表中的条件联接

我有一个包含4个表的简单查询。它工作正常，现在我需要在该查询中再添加一个联接，但这次表会根据该查询中的一个参数而变化

我的查询如下所示：

SELECT pat.Id,dgroup.Diagnosis as 'Description', ntest.Testname as 'TestName',
       dgroup.Id as 'ProrityId', subt.TestSubType, **ntest.SubGroupId**
  from pat.tbl_PatientTestRecord pat
          JOIN tbl_Test_DiagnosisGroup InTest on pat.IndividualTestId=InTest.Id
          JOIN tbl_NrlTests nTest ON InTest.TestId=nTest.id
          JOIN tbl_DiagnosisGroup dgroup ON Intest.DGId=dgroup.id
          Left Join 
             **tbl_vary**

• tbl\u变化取决于ntest.subseid。在

  tbl\u vary

  中可能会出现3个表

---

由于我的SQL有点弱，我将非常感谢为此提供适当的解决方案。

一种可能是联合子查询：

SELECT pat.Id,dgroup.Diagnosis as 'Description', ntest.Testname as 'TestName',
       dgroup.Id as 'ProrityId', subt.TestSubType, **ntest.SubGroupId**
  FROM pat.tbl_PatientTestRecord pat
  JOIN tbl_Test_DiagnosisGroup InTest on pat.IndividualTestId=InTest.Id
  JOIN tbl_NrlTests nTest ON InTest.TestId=nTest.id
  JOIN tbl_DiagnosisGroup dgroup ON Intest.DGId=dgroup.id
  LEFT JOIN
       (SELECT v1.SubGroupId, v1.Relevant_Col_1 AS rc1, v1.Relevant_Col_2 AS rc2
          FROM table_variant_1 AS v1
         WHERE v1.SubGroupId = ntest.SubGroupID
        UNION
        SELECT v2.SubGroupId, v2.Alternative_1 AS rc1, v2.Alternative_2 AS rc2
          FROM table_variant_2 AS v2
         WHERE v2.SubGroupId = ntest.SubGroupID
        UNION
        SELECT v3.SubGroupId, v3.Other_Source_1 AS rc1, v3.Other_Source_2 AS rc2
          FROM table_variant_3 AS v3
         WHERE v3.SubGroupId = ntest.SubGroupID
       ) AS u
    ON u.SubGroupId = ntest.SubGroupId

---

在子查询中不使用WHERE子句可能会更好，或者可能有其他方法来指定与UNION查询的联接……对于如何构造子查询，您还没有提供那么多线索。

您使用的是什么RDBMS？您所问的问题非常具体。@peterm我使用的是sql server 2012。。