Question

我有一个使用This Link编写的查询。

SQL Fiddle，以防上述链接过期。

示例代码：

;WITH MentorTable (CommonId, MentorId, MentorName) AS
(
    SELECT ROW_NUMBER() OVER(ORDER BY MentorId) AS CommonId,MentorId, MentorName
    FROM Mentor
)
,MenteeTable (CommonId, MenteeId, MenteeName) AS
(
    SELECT ROW_NUMBER() OVER(ORDER BY MenteeId) AS CommonId,MenteeId, MenteeName
    FROM Mentee
)
SELECT MR.MentorId,MR.MentorName,ME.MenteeId,ME.MenteeName
FROM MentorTable MR
   LEFT OUTER JOIN MenteeTable ME ON MR.CommonId = ME.CommonId

我想从五个表中获取数据并加入它们，而它们之间没有任何公共列。以上查询帮助我实现了这一目标。但是，在上面的查询中，Mentortable（来自表）将始终具有比其他表更多的记录，因此其他表的所有记录将在剩余行上显示为null。

然而，在我的五个表中，我不知道哪个表会有更多的记录。我希望我的查询从该表中进行选择，以便其余表中的任何行都不会错过。此外，我必须相应地更改我的连接表，以便所有其余的表除了＆＃34; from table＆＃34;加入了。任何帮助将不胜感激。

Answer 1

一种方法是full outer join。在您的情况下，这看起来像：

WITH MentorTable (CommonId, MentorId, MentorName) AS
(
    SELECT ROW_NUMBER() OVER(ORDER BY MentorId) AS CommonId,MentorId, MentorName
    FROM Mentor
)
,    MenteeTable (CommonId, MenteeId, MenteeName) AS
(
    SELECT ROW_NUMBER() OVER(ORDER BY MenteeId) AS CommonId,MenteeId, MenteeName
    FROM Mentee
)
SELECT COALESCE(MR.MentorId, ME.MentorId) as MentorId, MR.MentorName, ME.MenteeId, ME.MenteeName
FROM MentorTable MR FULL OUTER JOIN
     MenteeTable ME
     ON MR.CommonId = ME.CommonId;

然而，当您添加更多条件时，这会变得很麻烦。 on子句最终看起来像：

ON M5.CommonId = coalesce(M1.CommonId, M2.CommonId, M3.CommonId, M4.CommonId)

因此，您通常可以使用union all和group by处理此问题：

select CommonId, MAX(MentorName) as MentorName, MAX(MenteeName) as MenteeName
from ((select MR.CommonId, MR.MentorName, NULL as MenteeName from MentorTable MR) union all
      (select ME.CommonId, NULL, ME.MenteeName from MentorTable ME)
     ) t
group by CommonId;

根据条件确定将来自哪个表以及哪个表将加入

1 个答案: