mysql 按日期为每个用户选择最近的行

Question

我试图为每个唯一的用户 ID 选择最新的行，其中 pid = 50 且 active = 1。我一直无法弄清楚。

这是一个示例表

+-----+----------+-------+-----------------------+---------+
| 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       |
+-----+----------+-------+-----------------------+---------+

期望的结果

+-----+----------+-------+-----------------------+---------+
| 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       |
+-----+----------+-------+-----------------------+---------+

我已经尝试了很多东西，这是我得到的最接近的东西，但不幸的是它并没有停止。

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

计划

1. get last record grouping by userid where pid is 50 and is active
2. inner join to mytable to get the record info associated with last

<小时/>

查询

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
;

输出

+----+--------+-----+------------------------+--------+
| 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

注释

按照@Strawberry 的建议，更新为也在 pid 和 active 上加入。这将避免出现不活动或 pid 不是 50 但具有完全相同日期的记录也被渲染的可能性。