MySQL - Select to retrieve last datetime - mysql

Question

I've got a table in MySQL:
| period_duration | duration | sample | corner | country | partner | ok_rate
+---------------------+----------+--------+----------+---------+-----------------------+-------------------------
| 2014-12-15 17:00:00 | 3600 | 1 | GRPS_INB | ARG | Charlie | 98 |
| 2014-12-15 17:00:00 | 3600 | 1 | GRPS_INB | DEU | Jack | 90 |
| 2014-12-15 17:00:00 | 3600 | 1 | GRPS_INB | NLD | Will | 100 |
| 2014-12-15 20:00:00 | 3600 | 1 | GRPS_INB | ARG | Charlie | 98 |
| 2014-12-15 20:00:00 | 3600 | 1 | GRPS_INB | DEU | Jack | 90 |
| 2014-12-15 20:00:00 | 3600 | 1 | GRPS_INB | NLD | Will | 100 |
+-----------------+-------------+------+-----+---------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+-------------+------+-----+---------------------+-------+
| period_duration | datetime | NO | PRI | 0000-00-00 00:00:00 | |
| duration | varchar(6) | YES | | NULL | |
| sample | varchar(2) | YES | | NULL | |
| corner | varchar(10) | YES | | NULL | |
| country | varchar(60) | NO | PRI | | |
| partner | varchar(60) | NO | PRI | | |
| ok_rate | int(8) | YES | | NULL | |
+-----------------+-------------+------+-----+---------------------+-------+
This table increases from hour to hour.
I need a shellscript that will check each line from the last hour and that has an "ok_rate" lower than a given value, and return them to me on a select.
The select should bring country, the partner, and the ok_rate.
Example:
Let's say I want it to be done with <=98%¨on "ok_rate".
My returning rows would be:
| 2014-12-15 20:00:00 | 3600 | 1 | GRPS_INB | ARG | Charlie | 98 |
| 2014-12-15 20:00:00 | 3600 | 1 | GRPS_INB | DEU | Jack | 90 |
So far, I could only make it bring me the values below 98%, but not those that are also the last datetime that exists:
select country, partner, ok_rate from TABLE where ok_rate <= '98'
But how can I add a WHERE clause the makes it understand I want the last existing datetime that exists on period_duration too?
I mean something like:
SELECT country, partner, ok_rate FROM TABLE WHERE pdp_in_ok_rate <=98 AND (period_duration IS THE LAST ONE)?

Answer 1

This query should return what you want:
SELECT country, partner, ok_rate
FROM TABLE WHERE ok_rate <= 98
AND period_duration = (SELECT MAX(period_duration)
FROM TABLE)

Answer 2

Look in to GROUP BY for more detail because I'm not exactly sure what you want but something like this should help:
SELECT country, partner, ok_rate FROM TABLE
WHERE ok_rate <= '98'
GROUP BY `partner`, `country`
HAVING period_duration = MAX(period_duration);