mysql - 使用 GROUP BY、ORDER BY、GROUP_CONCAT 获取最旧的行

Question

我正在尝试使用以下语句来获取登录用户的消息列表。问题是尽管使用组和顺序，但它并不总是显示最新的消息。有人能解释一下吗？我应该使用子查询吗？

SELECT 
  messages.message_id,
  messages.message_parent_id,
  messages.message_project_id,
  messages.message_from_id,
  messages.message_from_email,
  messages.message_subject,
  messages.message_body,
  messages.message_sent_datetime,
  CONCAT(
    u_from.forename,
    ' ',
    u_from.surname
  ) AS from_x,
  GROUP_CONCAT(
    CONCAT(
      u_to.user_id,
      '||',
      u_to.forename,
      ' ',
      u_to.surname
    )
  ) AS to_x,
  m_recipients.recipient_message_read 
FROM
  messages 
  INNER JOIN users AS u_from 
    ON messages.message_from_id = u_from.user_id 
  INNER JOIN message_recipients AS m_recipients 
    ON messages.message_id = m_recipients.recipient_message_id 
  INNER JOIN users AS u_to 
    ON m_recipients.recipient_user_id = u_to.user_id 
WHERE (
    m_recipients.recipient_user_id = '$current_user_id' 
    OR messages.message_from_id = '$current_user_id'
  ) 
  AND messages.message_project_id = '$project_id' 
GROUP BY messages.message_parent_id 
ORDER BY messages.message_sent_datetime DESC 

fiddle :http://sqlfiddle.com/#!9/7fbfa/3

Answer 1

你确定不需要吗？

按 messages.message_parent_id、message.message_id 分组

如果您确定将 messages.message_sent_datetime 替换为

MAX(messages.message_sent_datetime) as message_sent_datetime,
...
ORDER BY message_sent_datetime

或

MIN(messages.message_sent_datetime) as message_sent_datetime,
...
ORDER BY message_sent_datetime

好的，试试 this way :

SELECT 
  messages.message_id,
  messages.message_parent_id,
  messages.message_project_id,
  messages.message_from_id,
  messages.message_from_email,
  messages.message_subject,
  messages.message_body,
  MAX(messages.message_sent_datetime) AS message_sent_datetime,
  messages.message_important,
  u_from.from_x,
  GROUP_CONCAT(u_to.to_x),
  m_recipients.recipient_message_read 
FROM
  messages 
  INNER JOIN (
    SELECT 
      user_id,
      CONCAT(forename,' ',surname) AS from_x
    FROM users 
    ) AS u_from 
    ON messages.message_from_id = u_from.user_id 
  INNER JOIN message_recipients AS m_recipients 
    ON messages.message_id = m_recipients.recipient_message_id 
  INNER JOIN 
     (SELECT
        user_id,
        CONCAT(user_id,'||',forename,' ',surname) AS to_x
      FROM users 
     ) AS u_to 
    ON m_recipients.recipient_user_id = u_to.user_id 
WHERE (
    m_recipients.recipient_user_id = 1
    OR messages.message_from_id = 1
  ) 
  AND messages.message_project_id = 1
GROUP BY messages.message_parent_id
ORDER BY messages.message_sent_datetime DESC