mysql subquery not producing all results - mysql

Question

I have two tables: contacts and client_profiles. A contact has many client_profiles, where client_profiles has foreign key contact_id:
contacts:
mysql> SELECT id,first_name, last_name FROM contacts;
+----+-------------+-----------+
| id | first_name | last_name |
+----+-------------+-----------+
| 10 | THERESA | CAMPBELL |
| 11 | donato | vig |
| 12 | fdgfdgf | gfdgfd |
| 13 | some random | contact |
+----+-------------+-----------+
4 rows in set (0.00 sec)
client_profiles:
mysql> SELECT id, contact_id, created_at FROM client_profiles;
+----+------------+---------------------+
| id | contact_id | created_at |
+----+------------+---------------------+
| 6 | 10 | 2014-10-09 17:17:43 |
| 7 | 10 | 2014-10-10 11:38:01 |
| 8 | 10 | 2014-10-10 12:20:41 |
| 9 | 10 | 2014-10-10 12:24:19 |
| 11 | 12 | 2014-10-10 12:35:32 |
+----+------------+---------------------+
I want to get the latest client_profiles for each contact. That means There should be two results. I want to use subqueries to achieve this. This is the subquery I came up with:
SELECT `client_profiles`.*
FROM `client_profiles`
INNER JOIN `contacts`
ON `contacts`.`id` = `client_profiles`.`contact_id`
WHERE (client_profiles.id =
(SELECT `client_profiles`.`id` FROM `client_profiles` ORDER BY created_at desc LIMIT 1))
However, this is only returning one result. It should return client_profiles with id 9 and 11.
What is wrong with my subquery?

Answer 1

It looks like you were trying to filter twice on the client_profile table, once in the JOIN/ON clause and another time in the WHERE clause.
Moving everything in the where clause looks like this:
SELECT `cp`.*
FROM `contacts`
JOIN (
SELECT
`client_profiles`.`id`,
`client_profiles`.`contact_id`,
`client_profiles`.`created_at`
FROM `client_profiles`
ORDER BY created_at DESC
LIMIT 1
) cp ON `contacts`.`id` = `cp`.`contact_id`
Tell me what you think.

Answer 2

Should be something like maybe:
SELECT *
FROM `client_profiles`
INNER JOIN `contacts`
ON `contacts`.`id` = `client_profiles`.`contact_id`
GROUP BY `client_profiles`.`contact_id`
ORDER BY created_at desc;
http://sqlfiddle.com/#!2/a3f21b/9

Answer 3

You need to prequery the client profiles table grouped by each contact.. From that, re-join to the client to get the person, then again to the client profiles table based on same contact ID, but also matching the max date from the internal prequery using max( created_at )
SELECT
c.id,
c.first_name,
c.last_name,
IDByMaxDate.maxCreate,
cp.id as clientProfileID
from
( select contact_id,
MAX( created_at ) maxCreate
from
client_profiles
group by
contact_id ) IDByMaxDate
JOIN contacts c
ON IDByMaxDate.contact_id = c.id
JOIN client_profiles cp
ON IDByMaxDate.contact_id = cp.contact_id
AND IDByMaxDate.maxCreate = cp.created_at