从SQL中选择唯一值

Question

好的，所以我试图从MySQL中选择唯一的值，但我并没有成功。所有用户

查询

SELECT DISTINCT private_messages.*, 
        users.ID AS userID, 
        users.username AS username, 
        profiles.img_url AS profileImage 
        FROM private_messages 
        INNER JOIN users ON users.username = private_messages.receiver 
        INNER JOIN profiles ON profiles.userID = users.ID 
       WHERE private_messages.sender ='Admin' 
       ORDER BY private_messages.sent_at DESC 

现在的问题是收到

基本上我试图让最后一封邮件的用户，我想，只选择接收最后的消息，不是所有的人

当前结果

我想实现什么？

• 只有消息新显示。我不能使用限制1，因为我得到所有用户的信息（从所有用户发送）

Answer 1

您shoudd筛选MAX（ID）

SELECT DISTINCT private_messages.*, 
       users.ID AS userID, 
       users.username AS username, 
       profiles.img_url AS profileImage 
       FROM private_messages 
       INNER JOIN users ON users.username = private_messages.receiver 
       INNER JOIN profiles ON profiles.userID = users.ID 
      WHERE private_messages.sender ='Admin' 
      AND private_messages.id in (select max(id) 
        from private_messages group by sender 
    )     
    ORDER BY private_messages.sent_at DESC 

Answer 2

如果我理解正确的，如果你有用户bla为接收者用户，您需要获取所有用户为他发送的所有最新消息。

如果是的话，如果你想“从所有用户收到的最后一封邮件用户”试试这个

SELECT private_messages.*, 
      users.ID AS userID, 
      users.username AS username, 
      profiles.img_url AS profileImage 
FROM private_messages INNER JOIN (
    SELECT MAX(id) as maxid from private_messages 
    where receiver = 'bla' 
    group by sender 
) m 
ON private_messages.id = m.maxid 
INNER JOIN users ON users.username = private_messages.sender 
INNER JOIN profiles ON profiles.userID = users.ID 
ORDER BY private_messages.sent_at DESC