mysql select most recent row by date for each user - mysql

Question

I'm trying to select the most recent rows for every unique userid where pid = 50 and active = 1. I haven't been able to figure it out.
Here is a sample table
+-----+----------+-------+-----------------------+---------+
| id | userid | pid | start_date | active |
+-----+----------+-------+-----------------------+---------+
| 1 | 4 | 50 | 2015-05-15 12:00:00 | 1 |
| 2 | 4 | 50 | 2015-05-16 12:00:00 | 1 |
| 3 | 4 | 50 | 2015-05-17 12:00:00 | 0 |
| 4 | 4 | 51 | 2015-06-29 12:00:00 | 1 |
| 5 | 4 | 51 | 2015-06-30 12:00:00 | 1 |
| 6 | 5 | 50 | 2015-07-05 12:00:00 | 1 |
| 7 | 5 | 50 | 2015-07-06 12:00:00 | 1 |
| 8 | 5 | 51 | 2015-07-08 12:00:00 | 1 |
+-----+----------+-------+-----------------------+---------+
Desired Result
+-----+----------+-------+-----------------------+---------+
| id | userid | pid | start_date | active |
+-----+----------+-------+-----------------------+---------+
| 2 | 4 | 50 | 2015-05-16 12:00:00 | 1 |
| 7 | 5 | 50 | 2015-07-06 12:00:00 | 1 |
+-----+----------+-------+-----------------------+---------+
I've tried a bunch of things and this is the closest I got but unfortunately it is not quit there.
SELECT *
FROM mytable t1
WHERE
(
SELECT COUNT(*)
FROM mytable t2
WHERE
t1.userid = t2.userid
AND t1.start_date < t2.start_date
) < 1
AND pid = 50
AND active = 1
ORDER BY start_date DESC

Answer 1

plan
get last record grouping by userid where pid is 50 and is active
inner join to mytable to get the record info associated with last
query
select
my.*
from
(
select userid, pid, active, max(start_date) as lst
from mytable
where pid = 50
and active = 1
group by userid, pid, active
) maxd
inner join mytable my
on maxd.userid = my.userid
and maxd.pid = my.pid
and maxd.active = my.active
and maxd.lst = my.start_date
;
output
+----+--------+-----+------------------------+--------+
| id | userid | pid | start_date | active |
+----+--------+-----+------------------------+--------+
| 2 | 4 | 50 | May, 16 2015 12:00:00 | 1 |
| 7 | 5 | 50 | July, 06 2015 12:00:00 | 1 |
+----+--------+-----+------------------------+--------+
sqlfiddle
notes
as suggested by #Strawberry, updated to join also on pid and active. this will avoid the possibility of a record which is not active or not pid 50 but has exact same date also being rendered.