Removing duplicate results from MySQL query - mysql

Question

I have two mysql tables Users & Messages. Please will you help me to create an sql query which returns only the most recent message either sent or received by the logged in user Mike ( user_id=1 ) along with the message_id, message_text & first_name of the person who the message was sent to or received from. Ordered by message_id in descending order.
So far i have the query:-
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
Which gives the result in the image below. It seems the ORDER BY is being ignored
Thank you in advance
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 |
----------------------------------------------------------------
Desired result
----------------------------------------------
| message_id | first_name | message_text |
----------------------------------------------
| 10 | George | Howdy |
| 9 | Sarah | Hello dear |
| 5 | Peter | Good day |
| 4 | John | Greetings |
----------------------------------------------
Database is available here

Answer 1

Just adding a GROUP BY clause may solve your problem, try this...
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: Changing the whole query
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
RESULT:
+------------+------------+--------------+
| message_id | first_name | message_text |
+------------+------------+--------------+
| 10 | George | Howdy |
| 9 | Sarah | Hello dear |
| 5 | Peter | Good Day |
| 4 | John | Greetings |

Answer 2

If you GROUP BY the field which you want to de-duplicate on. In this case GROUP BY tbl_users.first_name should do the trick.

Answer 3

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;

Answer 4

I am not sure you can achieve this through join. A sub select query may help you.
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;