MySQL: Is there a way to eliminates duplicate entries from the result - mysql

Question

I have a table named "message" that stores messages from one user to another user. I want to make a message box that contains both incoming and outcoming messages for particular user. This message box should be contain the last message between two users. So, I have to eliminate duplicate messages between two users. I tried group by and it eliminates duplicate messages but I don't pick the most recent message because order by works after group by. I tried distinct function to eliminates duplicate messages. It works well, but I have to select all colums which isn't possible with distinct
My message table:
+-------+---------+------+-----+------------+
| id | from_id | to_id| text| created_at |
+-------+---------+------+-----+------------+
| 1 | 1 | 2 | mes | 2014-01-16 |
| 2 | 2 | 1 | mes | 2014-01-17 |
| 3 | 1 | 3 | mes | 2014-01-18 |
| 4 | 3 | 1 | mes | 2014-01-19 |
+-------+---------+------+-----+------------+
My Group By SQL
SELECT * FROM message WHERE (from_id = 1 OR to_id = 1) GROUP BY(from_id + to_id) ORDER BY created_at DESC;
And Distinct
SELECT DISTINCT(from_id + to_id) FROM message WHERE (from_id = 1 OR to_id = 1)
In the above example, I want to select second and fourth message.
Is there a way to eliminate duplicate messages between two user from the result?
EDIT: I've improved the example

Answer 1

I tried group by and it eliminates duplicate messages but I don't pick the most recent message because order by works after group by
So you can order it before:
SELECT *
FROM (SELECT * FROM message ORDER BY created_at DESC)
WHERE (from_id = 1 OR to_id = 1) GROUP BY(from_id + to_id);

Answer 2

If I understand correctly what you're trying to achieve, you can leverage LEAST(), GREATEST() functions and non-standard GROUP BY extension behavior in MySQL like this
SELECT id, from_id, to_id, text, created_at
FROM
(
SELECT id, from_id, to_id, text, created_at
FROM message
ORDER BY LEAST(from_id, to_id), GREATEST(from_id, to_id), created_at DESC
) q
GROUP BY LEAST(from_id, to_id), GREATEST(from_id, to_id)
That will give you the last message row for each pair of users.
Output:
+------+---------+-------+------+------------+
| id | from_id | to_id | text | created_at |
+------+---------+-------+------+------------+
| 2 | 2 | 1 | mes | 2014-01-17 |
| 4 | 3 | 1 | mes | 2014-01-19 |
+------+---------+-------+------+------------+
Here is SQLFiddle demo

Answer 3

You can use:
ORDER BY id DESC LIMIT 1
or by timestamp (assuming it contains data AND time):
ORDER BY create_at DESC LIMIT 1
This will sort all results in a descending order and only give you the last row.
Hope this helps!

Answer 4

Just use a simple select. There is no reason for duplicates to be created.
SELECT from_id, to_id, text, created_at
FROM message
WHERE
(from_id = ? AND to_id = ??)
OR (from_id = ?? AND to_id = ?)
Here ? represents one id and ?? the other.
There would be no duplicates here. Ordering can be achieved in a few ways:
Order by most recent message regardless of sender:
SELECT from_id, to_id, text, created_at
FROM message
WHERE
(from_id = ? AND to_id = ??)
OR (from_id = ?? AND to_id = ?)
ORDER BY created_at DESC
Order all sender message first (then by created_at)
SELECT from_id, to_id, text, created_at
FROM message
WHERE
(from_id = ? AND to_id = ??)
OR (from_id = ?? AND to_id = ?)
ORDER BY from_id = ? DESC, created_at DESC

Answer 5

Try adding a HAVING clause after your GROUP BY: HAVING COUNT(*) > 1
or
SELECT
columns names, COUNT(*)
FROM
(SELECT DISTINCT
column names
FROM
message
)
message
GROUP BY
column names
HAVING COUNT(*) > 1

Answer 6

SQL Fiddle
MySQL 5.5.32 Schema Setup:
CREATE TABLE message
(`id` int, `from_id` int, `to_id` int, `text` varchar(3), `created_at` datetime)
;
INSERT INTO message
(`id`, `from_id`, `to_id`, `text`, `created_at`)
VALUES
(1, 1, 2, 'mes', '2014-01-16 00:00:00'),
(2, 2, 1, 'MUL', '2014-01-17 00:00:00')
;
Query 1:
SELECT *
FROM message
WHERE from_id = 1 OR to_id = 1
ORDER BY created_at DESC
limit 1
Results:
| ID | FROM_ID | TO_ID | TEXT | CREATED_AT |
|----|---------|-------|------|--------------------------------|
| 2 | 2 | 1 | MUL | January, 17 2014 00:00:00+0000 |