Question

I know this question has been asked many times, but I am having trouble implementing it. MySQL isn’t my greatest strength here.

I’ve built an email system for my site and what I want to do is when a user views their inbox they see their messages, however they see the latest sender, date and user details for that particular message instead of the original sender, date and user details.

My table structure as follows:

Message_Header:

-----------------------
messageid    INT
type         VARCHAR
subject      VARCHAR

Message_Recipient:

-----------------------
id           INT
messageid    INT
userid       INT
isread       ENUM
isspam       ENUM
isdelete     ENUM

Message_Detail:

--------------------
dtlid        INT
messageid    INT
from_userid  INT
hash         VARCHAR
type         SMALLINT
body         TEXT
title        VARCHAR
subtitle     VARCHAR
content      TEXT
thumb        VARCHAR
url          TEXT
images       VARCHAR
time_sent    DATETIME
timestamp    INT

I thought something like this would work:

select mh.messageid, mh.subject, mh.type, mr.isread, msg_dtl.from_userid, msg_dtl.firstname, msg_dtl.lastname, msg_dtl.gender, msg_dtl.avatar, msg_dtl.hash, msg_dtl.body, msg_dtl.time_sent
from message_header mh
inner join message_recipient mr on mr.messageid = mh.messageid  
inner join (
    select md.messageid, md.from_userid, u.firstname, u.lastname, u.gender, u.avatar, md.hash, md.body, md.time_sent
    from message_detail md
    inner join users u on u.userid = md.from_userid
    order by md.time_sent desc
    limit 1
) as msg_dtl ON mh.messageid = msg_dtl.messageid
where mr.userid = 5
and mr.isspam = '0'
and mr.isdelete = '0'

But apparently it limits the overall query to just 1 row!

I’ve looked around extensively and I’ve cobbled this together, but I’m worried its not optimised and will be a hog:

SELECT msg_dtl.type, msg_dtl.subject, msg_dtl.isread, u.firstname, u.lastname, u.gender, u.avatar, md.body, md.timestamp 
FROM message_detail md
INNER JOIN users u on u.userid = md.from_userid     
INNER JOIN
(
    SELECT mr.userid, mh.type, mh.subject, mr.isspam, mr.isdelete, mr.isread, md.messageid, md.body, max(timestamp) as timestamp
    FROM message_detail md
    inner join message_header mh on mh.messageid = md.messageid
    inner join message_recipient mr on mr.messageid = mh.messageid  
    GROUP BY md.messageid
) as msg_dtl
USING (timestamp, messageid)
where msg_dtl.userid = 5
and msg_dtl.isspam = '0'
and msg_dtl.isdelete = '0'

I would appreciate any pointers or someone optimising this. if you need any more info please let me know!

Answer 1

This might help with one exception… the IsRead flag.

First, I’m starting with a “PreQuery” to get based on a single Message (joined to the detail table), the most recent Sequence within the message (via max( mr.id) ), AND the latest DETAIL entry created (assuming auto-incrementing details via Max( md.DtlID ) ) for only those records for the given user that are not spam and not deleted. So this gives us at MOST, one record per message ID.

That being said, we can now do a direct join back to the header on the message ID 1:1 join

Then, 1:1 join BACK TO the message recipient on the most recent sequence received… if a chain message is back and forth 10+ times, here is where I assumed you would have intended the most recent email RECEIVED associated WITH the message ID. So, THIS was the record we are getting the “IsRead” flag… even though someone could read a more recent, yet never read a prior in an email chain.

Next, 1:1 join to the MESSAGE DETAIL table (via Max( md.DtlID ) ) for the last message detail FOR the given message. We don’t have to join on the Message ID since we HAVE the DETAIL ID of the message directly…

Finally, our detail can then join to the users table to get the “From” user information.

Hope this helps, including the clarification on how to derive the result set…

SELECT STRAIGHT_JOIN
      PreQuery.MessageID,
      mr2.IsRead,
      mh.Type,
      mh.Subject,
      md.from_UserID,
      md.isread,
      md.body, 
      md.timestamp,
      u.firstname, 
      u.lastname, 
      u.gender, 
      u.avatar
   from
      ( SELECT
              mr.MessageID,
              MAX( mr.ID ) as LastMessageSequence,
              MAX( md.DtlID ) as LastDetail
           from
              Message_Recipient mr
                 join Message_Detail md
                    ON mr.messageid = md.messageid
           where
                  mr.userid = 5
              and mr.isspam = '0'
              and mr.isdelete = '0'
           group by
              mr.MessageID ) PreQuery

      JOIN Message_Recipient mr2
         ON PreQuery.LastMessageSequence = mr2.ID

      JOIN Message_Header mh
         ON PreQuery.MessageID = mh.MessageID

      JOIN Message_Detail md
         ON PreQuery.LastDetail = md.DtlID
         JOIN users u 
            on md.from_userid = u.userid

   order by 
      md.timestamp desc