Getting the latest 'role-switch' timestamp from a messages table - mysql

Question

Problem
I am looking at trying to get the lowest timestamp (earliest) after the 'side' has changed in a ticket conversation, to see how long it has been since the first reply to the latest message.
Example:
A (10:00) : Hello
A (10:05) : How are you?
B (10:06) : I'm fine, thank you
B (10:08) : How about you?
A (10:10) : I'm fine too, thank you <------
A (10:15) : I have to go now, see you around!
Now what I am looking for is the timestamp of the message indicated by the arrow. The first message after the 'side' of the conversation changed, in this case from user to support.
Example data from table "messages":
mid conv_id uid created_at message type
2750 1 3941 1341470051 Hello support
3615 1 3941 1342186946 How are you? support
4964 1 2210 1343588022 I'm fine, thank you user
4965 1 2210 1343588129 How about you? user
5704 1 3941 1344258743 I'm fine too, thank you support
5706 1 3941 1344258943 I have to go now, see you around! support
What I have tried so far:
select
n.nid AS `node_id`,
(
SELECT m_inner.created_at
FROM messages m_inner
WHERE m_inner.mid = messages.mid AND
CASE
WHEN MAX(m_support.created_at) < MAX(m_user.created_at) THEN -- latest reply from user
m_support.created_at
ELSE
m_user.created_at
END <= m_inner.created_at
ORDER BY messages.created_at ASC
LIMIT 0,1
) AS `latest_role_switch_timestamp`
from
node n
left join messages m on n.nid = messages.nid
left join messages m_user on n.nid = m_user.nid and m_user.type = 'user'
left join messages m_support on n.nid = m_support.nid and m_support.type = 'support'
GROUP BY messages.type, messages.nid
ORDER BY messages.nid, messages.created_at DESC
Preferred result:
node_id latest_role_switch_timestamp
1 1344258743
But this has not yielded any results for the subquery. Am I looking in the right direction or should I try something else? I don't know if this would be possible in mysql.
Also this uses a subquery, which, for performance reasons, is not ideal, considering this query will probably be used in overviews, meaning it would have to run that subquery for every message in the overview.
If you require any more information, please tell me, as I am at my wit's end

Answer 1

Join the table to a max-date summary of itself to get the messages of the last block, then use mysql's special group-by support to pick the first row from those for each conversation:
select * from (
select * from (
select m.*
from messages m
join (
select conv_id, type, max(created_at) last_created
from messages
group by 1,2) x
on x.conv_id = m.conv_id
and x.type != m.type
and x.last_created < m.created_at) y
order by created_at) z
group by conv_id
This returns the whole row that was the first message of the last block.
See SQLFiddle.
Performance will be pretty good, because there are no correlated subqueries.