Question

I have a simple web application that allows users to message each other, the user can view their inbox to see their messages. The inbox should be grouped by sender, sorted by date of the latest message from that sender and also by the inbox read status.

I thought my query was working fine however i have just noticed that the ordering of some messages are not correct! I am pretty sure its due to the group by returning any message… is there anyway around this?

Have a look at this fiddle:
http://sqlfiddle.com/#!2/565c6/1

The last message of the sorted inbox, actually has the latest message yet its at the bottom of the inbox view!

Answer 1

I think you’re looking for a group-wise max:

SELECT *
FROM who_messages wm1
WHERE wm1.sent = (
  SELECT MAX(sent)
  FROM who_messages wm2
  WHERE wm2.from_userid = wm1.from_userid
    AND wm2.to_userid = 41
)
ORDER BY wm1.is_read ASC, wm1.sent DESC;

Sql Fiddle

Or, without the correlated sub-query:

SELECT *
FROM who_messages wm1
JOIN (
  SELECT from_userid, MAX(sent) as sent
  FROM who_messages
  WHERE to_userid = 41
  GROUP BY from_userid
) wm2 ON wm1.from_userid = wm2.from_userid AND wm1.sent = wm2.sent
ORDER BY wm1.is_read ASC, wm1.sent DESC;

Sql Fiddle