在mysql中選擇多列的DISTINCT行

Question

我有一個名為sk_messages的表。它的結構如下：

msg_id     msg_from_user_id  msg_to_user_id  msg_text   msg_date     msg_status  

 1              12                14          hai...   23-12-2013      unread

 2              12                14         ....     ...             unread

 3              13                 14        ...      ..               unread

我的要求是我想要顯示當前用戶的所有消息，條件是發送者應該顯示單個消息，即使他發送了多條狀態為未讀的消息。也就是說，從上面的上下文中，單個消息應該顯示ID為12的用戶。我嘗試了以下查詢，但它不起作用。

SELECT DISTINCT (msg_from_user_id), msg_text, msg_date
 FROM sk_messages
 WHERE msg_to_user_id =  '$user_id'
 AND msg_status =  'unread'
 ORDER BY msg_date


$user_id is the id of the login user

Answer 1

嘗試按ID group用戶。

SELECT msg_text, msg_date
 FROM sk_messages
 WHERE msg_to_user_id =  '$user_id'
 AND msg_status =  'unread'
GROUP BY msg_from_user_id
 ORDER BY msg_date

測試代碼以獲取最新消息

'SELECT * FROM ( SELECT * FROM message WHERE user_id = 1 ORDER BY created DESC LIMIT 1) as msg  GROUP BY user_id '

Answer 2

試試這個：

SELECT 
  m1.msg_from_user_id, 
  m1.msg_text, 
  m1.msg_date
FROM sk_messages AS m1
INNER JOIN
(
   SELECT msg_from_user_id, MAX(msg_date) AS LatestDate
   FROM sk_messages
   WHERE msg_to_user_id =  '$user_id' 
     AND msg_status =  'unread'
   GROUP BY msg_from_user_id
) AS m2  ON m1.msg_from_user_id = m2.msg_from_user_id
        AND m1.msg_date         = m2.LatestDate
ORDER BY m1.msg_date;

SQL小提琴演示

Answer 3

用這個

SELECT msg_from_user_id,
       msg_text, msg_date
       FROM sk_messages
 WHERE msg_to_user_id =  '$user_id'
 AND msg_status =  'unread'
 ORDER BY msg_date  GROUP BY msg_from_user_id