左外聯接，左表無重復

Question

我有兩個表（假設“左”和“右”表）。 右表從無到有幾行與左表的每一行匹配。

我需要執行一個查詢，在該查詢中我可以以與LEFT OUTER JOIN相同的方式聯接兩個表，但左側表中的每個現有表僅獲得一行。 該行應該是對應於右表中最高ID的那一行。

NATURAL JOIN可以工作，但是我不會從左邊的表中獲取與右邊的任何行都不匹配的行。

Answer 1

嘗試這個：

 Select L.[ValuesfromLeftTable], ...
        R.[ValuesfromRightTable], ...
 From LeftTable as L
   Left Join RightTable as R
     On R.Id = (Select Max(id) 
                From RightTable
                Where FK = L.Id)

Answer 2

這就是我要做的：

select * 
  from `left` as l 
    LEFT JOIN `right` as r 
       on (l.id = r.left_id) 
group by l.id 
order by r.id

Answer 3

您可以使用GROUP BY子句僅顯示不同的記錄

Answer 4

SELECT Left.Field1, Left.Field2, ... Right.Field1, Right.Field2, ...
FROM tblLeft AS Left
LEFT JOIN (SELECT CommonID, Max([ID]) AS MaxID
    FROM tblRight
    GROUP BY CommonID) AS RightMax ON Left.CommonID = RightMax.CommonID
LEFT JOIN tblRight AS Right ON RightMax.MaxID = Right.[ID] AND Left.CommonID = Right.CommonID