Mysql Unique column pairs as A,B or B,A where C is higher - mysqli

Question

I have this table
id|id_user_send|id_user_receive|date
-------------------------------------
0 | 1 | 2 | 2017-04-06
0 | 2 | 1 | 2017-04-07
0 | 1 | 4 | 2017-04-07
0 | 1 | 4 | 2017-04-08
0 | 4 | 1 | 2017-04-09
0 | 1 | 2 | 2017-04-10
i want only unique couple id_user_send,id_user_receive or id_user_receive,id_user_send where date is higher.
I dont see how get this whit group by or select distinct i try number off combine but not work. Can you just give me a way i want understand not just answer please thank you so much to helper.
edit : for me the couple 1, 4 and 4,1 is same i juste want the higher date

Answer 1

Here is the query that will help you to get the max C from the all rows that have A,B or B,A.
Here columns used are ..
A = sender (id_user_send)
B = receiver (id_user_receive)
C = day (date)
Explanation:
Query consists of UNION of 2 queries
In First Query, tables T1 and T2 are rows with sender, receiver, concatenated string (sender,receiver) and their max(day). WHERE clause matches the concatenated string (sr) with reverse of the same string, this will get all the rows where sender and receiver combination is same i.e. (1-4 and 4-1). Using CASE..WHEN..END we can get the row that has max date.
Second Query covers corner scenarios rows i.e. rows that one way interactions between sender and receiver AND same sender and receiver.
Sample Data Preparation
CREATE TABLE stack(sender int, receiver int, day date);
-- rows with sender:receiver as 1-4 or 4-1 (TWO_WAY INTERACTION)
INSERT INTO stack VALUES(1, 4, TO_DATE('2017-04-06', 'yyyy-mm-dd'));
INSERT INTO stack VALUES(1, 4, TO_DATE('2017-04-08', 'yyyy-mm-dd'));
INSERT INTO stack VALUES(4, 1, TO_DATE('2017-02-06', 'yyyy-mm-dd'));
INSERT INTO stack VALUES(4, 1, TO_DATE('2017-01-06', 'yyyy-mm-dd'));
-- rows with sender:receiver as 1-1 or 2-1 (TWO_WAY INTERACTION)
INSERT INTO stack VALUES(1, 2, TO_DATE('2017-06-06', 'yyyy-mm-dd'));
INSERT INTO stack VALUES(1, 2, TO_DATE('2017-04-06', 'yyyy-mm-dd'));
INSERT INTO stack VALUES(2, 1, TO_DATE('2017-09-06', 'yyyy-mm-dd'));
INSERT INTO stack VALUES(2, 1, TO_DATE('2017-01-06', 'yyyy-mm-dd'));
-- rows with sender:receiver as 3-6 (CORNER SCENARIO : ONE_WAY INTERACTION)
INSERT INTO stack VALUES(3, 6, TO_DATE('2017-09-06', 'yyyy-mm-dd'));
INSERT INTO stack VALUES(3, 6, TO_DATE('2017-01-06', 'yyyy-mm-dd'));
-- rows with sender:receiver as 7-7 (CORNER SCENARIO : SELF INTERACTION)
INSERT INTO stack VALUES(7, 7, TO_DATE('2017-09-06', 'yyyy-mm-dd'));
INSERT INTO stack VALUES(7, 7, TO_DATE('2017-01-06', 'yyyy-mm-dd'));
Sample Data View
Query
SELECT DISTINCT
CASE
WHEN T1.maxDay >= T2.maxDay THEN T1.sender ELSE T2.sender
END as sender,
CASE
WHEN T1.maxDay >= T2.maxDay THEN T1.receiver ELSE T2.receiver
END as receiver,
CASE
WHEN T1.maxDay >= T2.maxDay THEN T1.maxDay ELSE T2.maxDay
END as maxDay
FROM
(SELECT sender, receiver, (sender || ',' || receiver) AS sr, MAX(day) AS maxDay
FROM stack
GROUP BY sender, receiver, (sender || ',' || receiver)) T1,
(SELECT sender, receiver, (sender || ',' || receiver) AS sr, MAX(day) AS maxDay
FROM stack
GROUP BY sender, receiver, (sender || ',' || receiver)) T2
WHERE T1.sr = REVERSE(T2.sr)
UNION
SELECT T1.sender, T1.receiver, T1.maxday
FROM
(SELECT sender, receiver, (sender || ',' || receiver) AS sr, MAX(day) AS maxDay
FROM stack
GROUP BY sender, receiver, (sender || ',' || receiver)) T1
WHERE T1.sr NOT IN
(SELECT DISTINCT (REVERSE(sender || ',' || receiver))
FROM stack)
Result
Let me know if you need more explanation for query.
I have provided SQL Fiddle also for you to play around with the query so you can understand it by executing it part by part.
http://sqlfiddle.com/#!4/2d6f7/3/0