Question

大家好，

我找到了类似的科目，但却得不到我想要的东西。

我有3张桌子：

MESSAGE（行：MID，CONTENT，SENTDATE）

RECEIVED（行：RID，MEMBRE，FK_RE_MESSAGE_ID（外键：message.ID））

GROUP（行：GID，MICROID，FK_GR_MESSAGE_ID（外键：message.ID））

我想获取每个组的最后一条消息，以获取会员ID 基于谷歌搜索，这是我尝试过的最后一个请求：

SELECT * 
FROM MESSAGE as msg
LEFT JOIN RECEIVED AS rcd ON msg.MID=rcd.FK_RE_MESSAGE_ID 
LEFT JOIN GROUP AS grp ON msg.MID=grp.FK_GR_MESSAGE_ID
INNER JOIN (SELECT MID, MAX( SENTDATE) AS lastdate
FROM MESSAGE
GROUP BY MID) MESSAGE ON msg.MID=MESSAGE.MID
WHERE (rcd .MEMBRE)*

这将返回所选组的任何组中的所有消息。

请有人帮帮我吗？

Answer 1

您只需修复上一个join条件：

SELECT * 
FROM MESSAGE as msg LEFT JOIN
     RECEIVED AS rcd
     ON msg.MID=rcd.FK_RE_MESSAGE_ID LEFT JOIN
     "GROUP" AS grp
     ON msg.MID=grp.FK_GR_MESSAGE_ID INNER JOIN
     (SELECT MID, MAX( SENTDATE) AS lastdate
      FROM MESSAGE
      GROUP BY MID
     ) MESSAGE
     ON msg.MID = MESSAGE.MID and msg.SENTDATE = MESSAGE.lastdate
WHERE (rcd .MEMBRE)*;

如果您要过滤数据并在MESSAGE(MID, SENTDATE)上设置索引，则以下内容应该会更好;

SELECT * 
FROM MESSAGE as msg LEFT JOIN
     RECEIVED AS rcd
     ON msg.MID = rcd.FK_RE_MESSAGE_ID LEFT JOIN
     "GROUP" AS grp
     ON msg.MID = grp.FK_GR_MESSAGE_ID
WHERE (rcd .MEMBRE)* and
      not exists (select 1
                  from message m2
                  where m2.mid = msg.mid and m2.sentdate > msg.sentdate
                 );

sql Left Join，最大值

大家好，

1 个答案: