Sql server 互斥查询 问题:

如何将生成互斥结果的IF…ELSE角色转换为生成相同结果的查询

用例 作为负责人（当前登录用户），我可以获得组织中的用户列表。如果我被分配到任何部门，那么用户列表是分配给相同部门的较小集合

例子 使用一个union，每个where子句中都添加了“if”条件：

SELECT du.userId
FROM dbo.DepartmentsUsers AS du
JOIN (  SELECT departmentId
        FROM dbo.DepartmentsUsers
        WHERE organizationId = @organizationId
          AND userId = @userId) AS j
    ON du.departmentId = j.departmentId
WHERE du.organizationId = @organizationId
AND EXISTS (SELECT * FROM dbo.DepartmentsUsers WHERE organizationId = @organizationId AND userId = @userId)
UNION
SELECT userId
FROM dbo.OrganizationsUsers
WHERE organizationId = @organizationId
AND NOT EXISTS (SELECT * FROM dbo.DepartmentsUsers WHERE organizationId = @organizationId AND userId = @userId)

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只需要一个结果集。并非两者兼而有之。业务案例是互斥的。解决方案似乎是某种左连接，如果负责人被分配到任何部门，则有条件地强制进行内部连接。

 SELECT du.userId as UserId,
            'Department' as UserType // remove this line unnecessary
      FROM dbo.DepartmentsUsers AS du
      JOIN (
            -- Get department IDs assigned to the principal.
            SELECT departmentId
            FROM dbo.DepartmentsUsers
            WHERE organizationId = @organizationId
              AND userId = @userId) AS j
        ON du.departmentId = j.departmentId
      WHERE du.organizationId = @organizationId

      UNION ALL

      -- Get all user IDs in the organization.
      SELECT userId as UserId,
             'Organization' as UserType // remove this line unnecessary
      FROM dbo.OrganizationsUsers
      WHERE organizationId = @organizationId

SELECT du.userId
FROM dbo.DepartmentsUsers AS du
JOIN (  SELECT departmentId
        FROM dbo.DepartmentsUsers
        WHERE organizationId = @organizationId
          AND userId = @userId) AS j
    ON du.departmentId = j.departmentId
WHERE du.organizationId = @organizationId
AND EXISTS (SELECT * FROM dbo.DepartmentsUsers WHERE organizationId = @organizationId AND userId = @userId)
UNION
SELECT userId
FROM dbo.OrganizationsUsers
WHERE organizationId = @organizationId
AND NOT EXISTS (SELECT * FROM dbo.DepartmentsUsers WHERE organizationId = @organizationId AND userId = @userId)