2

我有两个 mysql 表用户和消息。请您帮我创建一个 sql 查询，该查询仅返回由登录用户 Mike ( user_id=1 ) 发送或接收的最新消息以及消息被发送到的人的 message_id、message_text 和 first_name 或从......收到。按 message_id 降序排列。

到目前为止，我有一个查询： -

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

这给出了下图中的结果。似乎 ORDER BY 被忽略了

按结果分组

先感谢您

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

数据库在这里可用

4

4 回答 4

3

只需添加一个GROUP BY子句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

结果：

+------------+------------+--------------+
| message_id | first_name | message_text |
+------------+------------+--------------+
| 10         | George     | Howdy        |
| 9          | Sarah      | Hello dear   |
| 5          | Peter      | Good Day     |
| 4          | John       | Greetings    |
于 2012-12-05T16:40:43.990 回答
2

如果您GROUP BY要对其进行重复数据删除的字段。在这种情况下GROUP BY tbl_users.first_name应该做的伎俩。

于 2012-12-05T16:56:40.503 回答
0

You are missing the table name in the ORDER BY clause. Also I suggest using aliases on the tables. Thus, your query might look like this:

/* 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;

However, I think it would be better to get the sent and received messages results separately. So:

/* 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;
于 2012-12-05T17:33:11.763 回答
0

我不确定您是否可以通过加入来实现这一目标。子选择查询可能会对您有所帮助。

  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;
于 2012-12-05T17:06:14.047 回答