如何在连接三个表时以更少的行获取所有信息？

Question

有人可以帮助我编写Oracle SQL Query以用于以下场景：

1. 公司可以有多个部门。
2. 每个部门都可以有多名员工。
3. 员工不需要分配部门，但必须是公司ID。

这是我正在尝试的输出：



这是我到目前为止所尝试的查询：

SELECT C.id   Company_Id, 
       C.name Company_Name, 
       D.id   Department_Id, 
       D.name Department_Name, 
       E.id   Employee_Id, 
       E.name Employee_Name 
FROM   Company C 
       FULL OUTER JOIN Department D 
                    ON D.CompanyId = C.Id 
       FULL OUTER JOIN Employee E 
                    ON E.CompanyId = C.Id AND E.DepartmentId = D.Id
ORDER BY Company_Id,Department_Id,Employee_Id;

但它给出了这个输出：



修改

Sql fiddle：http://sqlfiddle.com/#!4/df238/3/0

Answer 1

诀窍是将ON E.CompanyId = C.Id AND E.DepartmentId = D.Id加入条件转换为
   ON E.CompanyId = C.Id AND ( E.DepartmentId = D.Id OR E.DepartmentId is null )

SELECT C.id   Company_Id, 
       C.name Company_Name, 
       D.id   Department_Id, 
       D.name Department_Name, 
       E.id   Employee_Id, 
       E.name Employee_Name 
FROM   Company C 
       FULL OUTER JOIN Department D 
                    ON D.CompanyId = C.Id 
       FULL OUTER JOIN Employee E 
                    ON E.CompanyId = C.Id AND ( E.DepartmentId = D.Id OR E.DepartmentId is null )
ORDER BY Company_Id,Department_Id,Employee_Id;

Demo