从MySQL查询中删除重复的结果

Question

我有两个mysql表用户＆amp;消息。请你帮我创建一个sql查询，它只返回登录用户Mike（user_id = 1）发送或接收的最新消息以及message_id，message_text＆amp;邮件发送或接收的人的first_name。由message_id按降序排序。

到目前为止，我有查询： -

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

结果如下图所示。似乎忽略了ORDER BY

提前谢谢

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          |
----------------------------------------------------------------

期望的结果

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

数据库可用here

Answer 1

只需添加GROUP BY条款就可以解决您的问题，试试这个......

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

更新：更改整个查询

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

<强>结果：

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

Answer 2

如果您GROUP BY要删除重复的字段。在这种情况下，GROUP BY tbl_users.first_name应该可以解决问题。

Answer 3

我不确定你是否可以通过加入实现这一目标。子选择查询可以帮助您。

  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。另外我建议在表格上使用别名。因此，您的查询可能如下所示：

/* 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，我认为最好分别获取已发送和已接收的消息结果。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;