SQL在左联接上获取最大ID字段 [英] SQL getting max id field on a LEFT JOIN

问题描述

我正在尝试从tblimage中提取与每个用户的tblimage中的maxid相对应的照片.目前，我正在从消息表中获取所有消息，并为发布消息的用户提供了一张随机照片，我希望该照片是最新上传的照片.按照现在的书写方式，它只是从桌子上拉出一张随机照片.有什么建议?

I am trying to pull the photo from tblimage that corresponds to the maxid in the tblimage for each user. Currently, I am getting all the messages from the message table and a random photo for the user that posted the message, I would like the photo to be the latest uploaded photo. the way its written now it just pulls a random photo from the table. any suggestions?

表结构如下:

消息:msgid，消息，user_id，event_id
tblimage:ID，照片，用户ID

messages: msgid, message, user_id, event_id
tblimage: id, photo, userid

SELECT messages.*, tblimage.photo, max(tblimage.id) 
        FROM messages LEFT JOIN tblimage ON messages.user_id = tblimage.userid 
        GROUP BY messages.msg_id, messages.user_id 
        ORDER BY messages.msg_id DESC, tblimage.id desc

推荐答案

尝试

SELECT messages.*, T2.photo
FROM messages
LEFT JOIN (SELECT userid, MAX(id) AS maxid
           FROM tblimages
           GROUP BY userid) AS T1
ON messages.user_id = T1.userid
LEFT JOIN tblimages AS T2
ON T2.id = T1.maxid
ORDER BY messages.msg_id DESC

会在tblimage中为每个用户找到max(id)，然后使用该ID将每个用户加入该用户的最新照片.

which finds max(id) for each user in tblimages, then uses that to join each user to the latest photo for that user.

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