SQL Server：基于多对多：n关系中的最新ID选择

Question

我需要做的是选择评论详细信息以及对该评论采取的最后一项操作； 我有3张桌子：

评论

CommentID, commentText, userID, date_posted

行动

ActionID, action_taken,userID,date_actioned

和CommentJoinAction

id,ActionID,CommentID

可能有一个评论，但评论上有许多动作。

我的SQL看起来像：

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*/

那么我能做些什么，以便我总是拿起最新的commentAction作为评论。

非常感谢

Answer 1

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

Answer 2

这是你想要的？

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 3

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 4

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，如果您需要所有“操作”字段：

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