CODEWITHSUNDEEP
mysql
boeing
boeing

Reputation: 302

Removing duplicate results from MySQL query

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

Group by result

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

Upvotes: 2

Views: 9357

Answers (4)

rajukoyilandy
rajukoyilandy

Reputation: 5471

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    |

Upvotes: 3

Yogendra Singh
Yogendra Singh

Reputation: 34367

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;

Upvotes: 0

Geo
Geo

Reputation: 12996

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;

Upvotes: 0

cjsissingh
cjsissingh

Reputation: 64

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.

Upvotes: 2

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