從SQL中選擇唯一值

Question

好的，所以我試圖從MySQL中選擇唯一的值，但我並沒有成功。所有用戶

查詢

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 

現在的問題是收到

基本上我試圖讓最後一封郵件的用戶，我想，只選擇接收最後的消息，不是所有的人

當前結果

我想實現什麼？

• 只有消息新顯示。我不能使用限制1，因爲我得到所有用戶的信息（從所有用戶發送）

Answer 1

您shoudd篩選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

如果我理解正確的，如果你有用戶bla爲接收者用戶，您需要獲取所有用戶爲他發送的所有最新消息。

如果是的話，如果你想「從所有用戶收到的最後一封郵件用戶」試試這個

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