MySQL左连接从一个表中选择最高值

Question

I have the next query:

SELECT chat.*, message.time as last_message, message.content as last_message_content 
FROM chat 
LEFT JOIN message ON chat.id = message.chat_id 
GROUP BY chat.id 
ORDER BY message.time ASC

now, I want to pick the newest message.time, but right now it gives me the first one..

any idea on how this could be accomplished?

thanks

Answer 1

Use a subquery to get the last message time for the chat and join that back to the message table:

SELECT c.*, m.time as last_message, m.content as last_message_content 
FROM chat c LEFT JOIN
     (select chat_id, max(time) as last_message_time
      from message
      group by chat_id
     ) ml
     ON c.id = ml.chat_id LEFT JOIN
     message m
     ON m.chat_id = ml.chat_id and m.time = ml.lst_message_time
ORDER BY m.time ASC;

Do not use group by the way you were using it. The columns from message come from indeterminate rows, so you have no control over which values are chosen.

Answer 2

按DESC顺序对其进行排序以获取最新消息。