SQL Server：根据多对n:m关系中的最新id进行选择

Question

我需要做的是选择评论详细信息以及对评论采取的最后操作；我有 3 个表：

Comment

CommentID, commentText, userID, date_posted

Action

ActionID, action_taken,userID,date_actioned

和 CommentJoinAction

id,ActionID,CommentID

可以有一个评论，但该评论可以有多个操作。

我的 SQL 看起来像这样：

Select /*snip comment details and such*/
From Comment
Inner Join (select max(actionid) from commentjoinaction) as cja on /*blah cause you know from reading this, it won't work*/

那么我可以做什么，以便我始终为评论获取最新的 commentAction 。

非常感谢

Answer 1

这是您要找的吗？

SELECT 
/*Select desired fields*/
FROM Comments AS C
    INNER JOIN (
                SELECT 
                    CommentID
                    ,MAX(ActionID) AS ActionID
                FROM CommentJoinAction
                GROUP BY CommentID
            )AS CJA
        ON C.CommentID = CJA.CommentID
        INNER JOIN ACTION AS A
            ON CJA.ActionID = A.ActionID

Answer 2

select C.*, A.* from Comment C
inner join 
(
    select CommentID, Max(ActionID) as LatestActionID from CommentJoinAction
    group by CommentID
) CJA on C.CommentID = CJA.CommentID
inner join Action A on CJA.LatestActionID = A.ActionID

Answer 3

如果您只想要 actionID

select c.*, (
  select max(actionID) 
  from CommentJoinAction cja 
  where cja.commentID = c.commentID
) as maxActionID
from Comment c

或者如果您想要所有 Action 字段：

select c.*, a.*
from Comment c 
inner join Action a 
  on a.actionID =     (
   select max(actionID) 
   from CommentJoinAction 
   where commentID = c.commentID
)

Answer 4

SELECT t.commentText, t.action_taken
    FROM (SELECT c.commentText, a.action_taken,
                 ROW_NUMBER() OVER (PARTITION BY c.CommentID ORDER BY a.date_actioned DESC) AS RowNum
              FROM Comment c
                  INNER JOIN CommentJoinAction cja
                      ON c.CommentID = cja.CommentID
                  INNER JOIN Action a
                      ON cja.ActionID = a.ActionID
          ) t
    WHERE t.RowNum = 1