Can't integrate INNER JOIN in my query - mysql

Question

Hello I'm using the query below to select the latest message from each unique conversation. Everything works perfectly except when I try to integrate an INNER JOIN to try to link the user's user_id with their username in the users table.
I've tried pretty much every combination to integrate the INNER JOIN so I decided to separate it from the working query until I get help.
I made a Fiddle http://sqlfiddle.com/#!9/a9bbc/1 I just want user_id on the right to print the user's username
SELECT message_id,
msg,
user_id
FROM messages
JOIN (SELECT user_id,
Max(dat) m
FROM ((SELECT message_id,
recipient_id user_id,
dat
FROM messages
WHERE owner_id = 1)
UNION
(SELECT message_id,
owner_id user_id,
dat
FROM messages
WHERE recipient_id = 1)) t1
GROUP BY user_id) t2
ON ( ( owner_id = 1
AND recipient_id = user_id )
OR ( owner_id = user_id
AND recipient_id = 1 ) )
AND ( dat = m )
ORDER BY dat DESC
.
users.username INNER JOIN users ON messages.user_id = users.user_id;

Answer 1

If messages.message_id is an AUTO_INCREMENT PRIMARY KEY column, this will probably be the fastest way to receive the desired result:
select m.message_id, m.msg, u.username,
case sub.other_user_id
when m.owner_id then 'received from'
when m.recipient_id then 'sent to'
end as direction
from (
select other_user_id, max(message_id) as message_id
from (
select recipient_id as other_user_id, max(message_id) as message_id
from messages
where owner_id = #uid
group by recipient_id
union all
select owner_id as other_user_id, max(message_id) as message_id
from messages
where recipient_id = #uid
group by owner_id
) sub
group by other_user_id
) sub
join messages m on m.message_id = sub.message_id
join users u on u.user_id = sub.other_user_id
order by sub.message_id desc
I've also added the column direction. This way you will know if the message has been sent or received. The result would be like:
| message_id | msg | username | direction |
|------------|------------------------------------------------------------------|----------|---------------|
| 9 | You should also see this | User3 | sent to |
| 8 | you should Now see this instead even with the owner_id flipped.. | User2 | received from |
http://sqlfiddle.com/#!9/a9bbc/34
Note that you can still add any column from the users and messages tables in the SELECT clause.
To get this query work fast you will need indexes on messages(owner_id, recipient_id) and messages(recipient_id, owner_id).

Answer 2

I think this is correct... finally. The not exists filters out responses after the start.. the inner query grabs the most recent conversation. This should match the expected output.
SELECT dT.maxid AS message_id
,(SELECT msg FROM messages M WHERE M.message_id = dT.maxid) AS message_id
,(SELECT username FROM users U WHERE U.user_id = dT.owner_id) AS user_id
FROM (
SELECT MAX(message_id) as maxid
,owner_id
,recipient_id
FROM messages M
GROUP BY owner_id, recipient_id
) AS dT
WHERE NOT EXISTS(SELECT *
FROM messages M2
WHERE M2.recipient_id = dT.owner_id
AND M2.owner_id = dT.recipient_id
AND M2.message_id > dT.maxid)
ORDER BY dT.maxid
Produces output:
message_id message_id user_id
8 you should Now see this instead even with the User2
owner_id flipped.. The users_id in the query is 1
and it gets the conversation with user_id 2
9 You should also see this User1
fiddle

Answer 3

When you have joined any tables or subqueries, you really really really need to include table name or table aliases with all column references. I changed the maximum dat to maxdat below so I could use m for messages.
SELECT
m.message_id
, m.msg
, m.user_id
FROM messages m
JOIN (
SELECT
user_id
, MAX(dat) maxdat
FROM (
(SELECT
message_id
, recipient_id user_id
, dat
FROM messages
WHERE owner_id = 1)
UNION
(SELECT
message_id
, owner_id user_id
, dat
FROM messages
WHERE recipient_id = 1)
) t1
GROUP BY
user_id
) t2 ON (
(m.owner_id = 1 AND m.recipient_id = t2.user_id)
OR (m.owner_id = t2.user_id AND m.recipient_id = 1)
)
AND (m.dat = t2.maxdat)
ORDER BY
m.dat DESC

Answer 4

You need to join your final result with users table and get the value of users.username based on user_id. Try this:
SELECT messages.message_id,
messages.msg,
users.user_id,
users.username
FROM users, messages
JOIN (SELECT user_id,
Max(dat) m
FROM ((SELECT message_id,
users.username,
recipient_id user_id,
dat
FROM messages
INNER JOIN users ON messages.recipient_id = users.user_id
WHERE owner_id = 1)
UNION
(SELECT message_id,
users.username,
owner_id user_id,
dat
FROM messages
INNER JOIN users ON messages.owner_id = users.user_id
WHERE recipient_id = 1)) t1
GROUP BY user_id) t2
ON ( ( owner_id = 1
AND recipient_id = user_id )
OR ( owner_id = user_id
AND recipient_id = 1 ) )
AND ( dat = m )
WHERE users.user_id = messages.user_id
ORDER BY dat DESC

Answer 5

If I understand correctly, you want the most recent messages where the send or recipient is "1" and associated user names.
That suggests:
select m.*, ur.username as recipient_name, us.username as sender_name
from messages m join
users ur
on m.recipient_id = ur.user_id join
users us
on m.sender_id = us.user_id
where m.date = (select max(m2.dat)
from messages m2
where 1 in (m2.recipient_id, m2.sender_id)
) and
1 in (m.recipient_id, m.sender_id);
EDIT:
Based on your SQL Fiddle the above understanding is not correct. You want to consider pairs of recipients, so I think you want:
select m.*, ur.username as recipient_name, us.username as sender_name
from messages m left join
users ur
on m.recipient_id = ur.user_id left join
users us
on m.owner_id = us.user_id
where m.dat = (select max(m2.dat)
from messages m2
where (m2.recipient_id, m2.owner_id) = (m.recipient_id, m.owner_id) or
(m2.recipient_id, m2.owner_id) = (m.owner_id, m.recipient_id)
) and
1 in (m.recipient_id, m.owner_id);
Here is the SQL Fiddle.
If the ordering of the pair is import (so (1, 2) is different from (2, 1) remove the second condition in the innermost where clause.