從MySQL查詢中刪除重複結果

Question

我有兩個mysql表用戶&消息。請你幫我創建一個sql查詢，它只返回登錄用戶Mike（user_id = 1）發送或接收到的最新消息，以及發送消息的人的message_id，message_text & first_name，或者從......收到。按降序排列的message_id排序。

到目前爲止，我有查詢： -

SELECT message_id, first_name, message_text FROM tbl_users, tbl_messages 
WHERE 
(tbl_users.user_id = tbl_messages.sender_id AND tbl_messages.receiver_id = 1 
OR tbl_users.user_id = tbl_messages.receiver_id AND tbl_messages.sender_id = 1) 
GROUP BY tbl_users.first_name 
ORDER BY message_id DESC 

其中給出的結果如下圖所示。看來ORDER BY被忽略

預先感謝您

tbl_users

--------------------------- 
| user_id | first_name | 
--------------------------- 
| 1  | Mike   | 
| 2  | John   | 
| 3  | George  | 
| 4  | Peter  | 
| 5  | Sarah  | 
--------------------------- 

tbl_messages

---------------------------------------------------------------- 
| message_id | sender_id | receiver_id | message_text | 
---------------------------------------------------------------- 
| 1   | 2   | 1    | Hello   | 
| 2   | 3   | 1    | How are you | 
| 3   | 1   | 5    | Hi there  | 
| 4   | 2   | 1    | Greetings  | 
| 5   | 1   | 4    | Good day  | 
| 6   | 3   | 1    | Hi    | 
| 7   | 5   | 1    | A message  | 
| 8   | 5   | 4    | Good morning | 
| 9   | 1   | 5    | Hello dear  | 
| 10   | 1   | 3    | Howdy   | 
---------------------------------------------------------------- 

期望的結果

---------------------------------------------- 
| message_id | first_name | message_text | 
---------------------------------------------- 
| 10   | George  | Howdy   | 
| 9   | Sarah  | Hello dear  | 
| 5   | Peter  | Good day  | 
| 4   | John  | Greetings  | 
---------------------------------------------- 

數據庫可用here

Answer 1

只要添加一個 GROUP BY條款可以解決您的問題，試試這個...

SELECT message_id, first_name, message_text FROM tbl_users, tbl_messages 
WHERE 
(tbl_users.user_id = tbl_messages.sender_id AND tbl_messages.receiver_id = 1 
OR tbl_users.user_id = tbl_messages.receiver_id AND tbl_messages.sender_id = 1) 
GROUP BY tbl_users.user_id 
ORDER BY message_id DESC 

UPDATE：更改整個查詢

SELECT t2.message_id, t2.first_name, t2.message_text FROM 
(SELECT u.user_id, m.message_id, u.first_name, m.message_text FROM `tbl_users` u 
    JOIN `tbl_messages` m ON (u.user_id = m.sender_id AND m.receiver_id = 1) 
    OR (u.user_id = m.receiver_id AND m.sender_id = 1) 
    ORDER BY m.message_id DESC 
) t2 
GROUP BY t2.user_id 
ORDER BY t2.message_id DESC 

結果：

+------------+------------+--------------+ 
| message_id | first_name | message_text | 
+------------+------------+--------------+ 
| 10   | George  | Howdy  | 
| 9   | Sarah  | Hello dear | 
| 5   | Peter  | Good Day  | 
| 4   | John  | Greetings | 

Answer 2

如果GROUP BY要刪除重複的領域。在這種情況下，GROUP BY tbl_users.first_name應該做到這一點。

Answer 3

我不確定你可以通過連接實現這一點。子選擇查詢可能會對您有所幫助。

select a.first_name, 
     (select message_text from tbl_messages b 
      where (a.user_id = b.sender_id and b.sender_id = 1) 
       or (a.user_id = b.receiver_id and b.receiver_id = 1) 
      order by message_id DESC 
      LIMIT 1) 
    from tbl_users a; 

Answer 4

您在ORDER BY子句中缺少表名。另外我建議在表格上使用別名。因此，您的查詢可能看起來像這樣：

/* All Mike's messages */ 
SELECT 
    m.message_id, 
    u.first_name, 
    m.message_text 
FROM 
    tbl_users AS u, 
    tbl_messages AS m 
WHERE 
(
    u.user_id = m.sender_id AND 
    m.receiver_id = 1 
) OR (
    u.user_id = m.receiver_id AND 
    m.sender_id = 1 
) 
ORDER BY 
    m.message_id DESC; 

但是，我認爲最好單獨獲取發送和接收的消息結果。所以：

/* Messages sent my Mike */ 
SELECT 
    m.message_id, 
    u.first_name, 
    m.message_text 
FROM 
    tbl_users AS u 
    INNER JOIN tbl_messages AS m ON m.receiver_id = u.user_id 
WHERE 
    m.sender_id = 1 
ORDER BY 
    m.message_id DESC; 

/* Messages received my Mike */ 
SELECT 
    m.message_id, 
    u.first_name, 
    m.message_text 
FROM 
    tbl_users AS u 
    INNER JOIN tbl_messages AS m ON u.user_id = m.sender_id 
WHERE 
    m.receiver_id = 1 
ORDER BY 
    m.message_id DESC;