How to join / union multiple results from same table? - mysql

Question

I've mysql tables that looks like :
user_messages
id | user_id | phone_number | message | direction | created_at
users
id | name
I want to 'group by' user_messages two times and UNION the result. Why I want to do this? because user_id sometimes has a valid user id (anything but '-1') then I group by it, if it has -1, then group by phone_number.
I also want to left join the result with users table to get the user name in case user_id is set to a valid user
I'm almost done with the query, but the problem is:
- I want the result to have the record that results from group by to be the latest one, which means, the biggest created_at value
select * from (
(
select *, count(*) as `total` from
(select `user_id`, `message`, `created_at`, `phone_number`,`direction` from `users_messages` where `user_id` != -1 order by `created_at` desc)
as `t1` group by `user_id`
)
union
(
select *, count(*) as `total` from
(select `user_id`, `message`, `created_at`, `phone_number`,`direction` from `users_messages` where `user_id` = -1 order by `created_at` desc)
as `t2` group by `phone_number`
)
) as `t3`
left join (select `id`,`name` from `users`) as `t4` on `t3`.`user_id` = `t4`.`id` order by `created_at` desc
What this gets me is the results not sorted by created_at DESC
Update:
The query actually works in my local machine but not on the production server. In my local machine I have 5.5.42 - Source distribution and in server Ver 14.14 Distrib 5.7.17, for Linux (x86_64) using EditLine wrapper ... What could be wrong?
In local machine it correctly returns me the max created_at but in server it returns the FIRST created for the grouped by record

Answer 1

Something like this should work:
SELECT s.`user_id`, um.`phone_number`, s.msgCount
, um.`message`, um.`created_at`, um.`direction`
, u.`name` AS userName
FROM (
SELECT `user_id`, IF(`user_id` = -1, `phone_number`, '') AS altID, MAX(`created_at`) AS lastCreatedAt, COUNT(*) AS msgCount
FROM `users_messages`
GROUP BY user_id, altID
) AS s
INNER JOIN `users_messages` AS um
ON s.user_id = um.user_id
AND s.altID = IF(um.`user_id` = -1, um.`phone_number`, '')
AND s.lastCreatedAt = um.created_at
LEFT JOIN `users` AS u
ON s.user_id = u.user_id
ORDER BY um.created_at DESC
;
The s subquery gets the summary information for each user and userless phone number; the summary information calculated includes the most recent created_at value for use in the following....
The join to um gets the row data for their last messages (by including the lastCreatedAt value from s in the join criteria)
The final join to users is used to get the user.name for the known users (and assumes there will be no -1 user, or that such a user would have an appropriate 'unknown' name.)

Answer 2

Since you're grouping by user_id and phone_number, you can't keep message or direction. Add a max function for created_at in each subquery. I think this would work.
select * from (
(
select user_id
,'' as phone_number
,max('created_at') as 'created_at'
,count(*) as `total` from
(select `user_id`
,`created_at`
from `users_messages`
where `user_id` != -1)
as `t1` group by `user_id`
)
union
(
select '' as user_id
,phone_number
,max('created at') as 'created_at'
,count(*) as `total` from
(select `created_at`
,`phone_number'
from `users_messages`
where `user_id` = -1)
as `t2` group by `phone_number`
)
) as `t3`
left join (select `id`,`name` from `users`) as `t4`
on `t3`.`user_id` = `t4`.`id`
order by `created_at` desc