Selecting first row that matches a criteria in Groupby - mysql

Question

Given the following messages table where channel is a particular chat session, and user1 and user2 are the users in the chat:
+---------+-------+-------+------------+
| channel | user1 | user2 | message |
+---------+-------+-------+------------+
| 5 | 15 | 8 | Hello |
| 5 | 15 | 8 | I'm John |
| 5 | 8 | 15 | Hi John |
| 6 | 9 | 15 | yo |
| 6 | 15 | 9 | heyo |
| 6 | 9 | 15 | you here? |
| 8 | 15 | 10 | Hi |
| 8 | 15 | 10 | you there? |
+---------+-------+-------+------------+
I'd like to group by the channel and select the first response row (the first row where the second person said something). If the second person never responded as in channel 8, then they don't need to show up in the output.
So my expected output would be this:
+---------+-------+-------+---------+
| channel | user1 | user2 | message |
+---------+-------+-------+---------+
| 5 | 8 | 15 | Hi John |
| 6 | 15 | 9 | heyo |
+---------+-------+-------+---------+
Note that there is a timestamp column, just forgot to include it. Any help would be appreciated, been searching all over for a solution an have yet to come up with any. Thanks.

Answer 1

Assuming that you have a timestamp column, you can get the user2 for the first message as:
select m.*
from messages m
where not exists (select 1
from messages m2
where m2.channel = m.channel and
m2.timestamp < m.timestamp
);
So, if you want the first message from this, you can use the group_concat()/substring_index()` trick:
select m.channel, m.user1, m.user2,
substring_index(group_concat(m2.messages order by m2.timestemp separator '|'), '|', 1)
from messages m join
(select m.*
from messages m
where not exists (select 1
from messages m2
where m2.channel = m.channel and
m2.timestamp < m.timestamp
)
) mfirst
on m.channel = mfirst.channel and
m.user1 = mfirst.user2
group by m.channel, m.user1, m.user2;

Answer 2

Not entirely convinced myself. Feel free to improve.
The limit 1 relies on the table being read top down every time.
And I suspect all the different selects can be done more elegantly.
But at least it gives the required result for the sample data :)
SELECT channelchat.channel,
(SELECT user2
FROM chat firstline
WHERE firstline.channel = channelchat.channel
LIMIT 1) seconduser,
(SELECT user1
FROM chat firstline
WHERE firstline.channel = channelchat.channel
LIMIT 1) firstuser,
(SELECT message
FROM chat secondline
WHERE secondline.channel = channelchat.channel
AND secondline.user1 = seconduser
LIMIT 1) response
FROM chat channelchat
GROUP BY channelchat.channel
HAVING response IS NOT NULL
sqlfiddle