Mysql 将平均值限制为最近的记录

我想将此查询限制为仅计算“学生”回答的最后10个问题的平均“持续时间”

i、 e.对于给定的学生，获取回答的问题列表，以及他们过去10次尝试该问题的平均时间

+----------+-----------+
| question | timeTaken |
+----------+-----------+
| 2        | 3956      |
+----------+-----------+
| 5        | 11102     |
+----------+-----------+
| 9        | 14527     |
+----------+-----------+

样本表数据

+---------+----------+----------+---------------------+
| student | question | duration | when                |
+---------+----------+----------+---------------------+
| 1       | 1        | 5301     | 2017-03-10 16:35:11 |
+---------+----------+----------+---------------------+
| 2       | 2        | 5217     | 2017-03-10 16:36:51 |
+---------+----------+----------+---------------------+
| 1       | 2        | 3459     | 2017-03-10 16:44:42 |
+---------+----------+----------+---------------------+
| 2       | 2        | 2694     | 2017-03-10 16:44:55 |
+---------+----------+----------+---------------------+
| 2       | 5        | 11102    | 2017-03-10 16:45:10 |
+---------+----------+----------+---------------------+
| 2       | 9        | 14527    | 2017-03-10 16:45:14 |
+---------+----------+----------+---------------------+
| 3       | 5        | 1763     | 2017-03-10 16:45:47 |
+---------+----------+----------+---------------------+

---

什么决定了“最后10次尝试”？发布一些示例数据。

last 10“question”

您指的是最后10个问题还是最后10个唯一问题？希望每个唯一问题返回一行。每行显示该问题最后10次尝试的平均值。

+---------+----------+----------+---------------------+
| student | question | duration | when                |
+---------+----------+----------+---------------------+
| 1       | 1        | 5301     | 2017-03-10 16:35:11 |
+---------+----------+----------+---------------------+
| 2       | 2        | 5217     | 2017-03-10 16:36:51 |
+---------+----------+----------+---------------------+
| 1       | 2        | 3459     | 2017-03-10 16:44:42 |
+---------+----------+----------+---------------------+
| 2       | 2        | 2694     | 2017-03-10 16:44:55 |
+---------+----------+----------+---------------------+
| 2       | 5        | 11102    | 2017-03-10 16:45:10 |
+---------+----------+----------+---------------------+
| 2       | 9        | 14527    | 2017-03-10 16:45:14 |
+---------+----------+----------+---------------------+
| 3       | 5        | 1763     | 2017-03-10 16:45:47 |
+---------+----------+----------+---------------------+