MYSQL查询：获取按数据分组的最新日期

Question

我有一张桌子聊天。

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消息，我senderID，我，ciptantID，我，日期

我想通过对话获取有关最新消息组的查询。 （例如，如果'senderID = 1且recipentID = 2'与'senderID = 2且recipentID = 1'是相同的对话）

Answer 1

您可以使用in。 MySQL支持in的元组，因此这是一种方法：

select c.*
from chats c
where (least(senderID, recipentID), greatest(senderID, recipentID), date) in
       (select least(senderID, recipentID), greatest(senderID, recipentID), max(date)
        from chats c
        group by least(senderID, recipentID), greatest(senderID, recipentID)
       );

Answer 2

• 您可以将两个人之间的特定对话识别为“ concatenated id-LEAST id”的GREATEST字符串
• 对“对话”进行分组以找到每个对话的Max日期（最后消息日期）。
• 将此作为Derived table结果集，并加入到主表（使用ON（显式）和WHERE（隐式连接的类型））

尝试以下查询：

SELECT 
  CONCAT(LEAST(t1.senderID, t1.recipentID), 
         '-', 
         GREATEST(t1.senderID, t1.recipentID)) AS t1_conversation, 
  t1.*
FROM chat_table AS t1 
JOIN ( 
      SELECT CONCAT(LEAST(t2.senderID, t2.recipentID), '-', 
                    GREATEST(t2.senderID, t2.recipentID)) AS conversation, 
             MAX(t2.date) as last_message_date 
      FROM chat_table AS t2 
      GROUP BY t2.conversation
     ) AS t3 ON t3.max_date = t1.date 
WHERE t1_conversation = t3.conversation