LeetCode MySQL 578. 查询回答率最高的问题

id 表示用户 id；
action 有以下几种值："show"，"answer"，"skip"；
当 action 值为 "answer" 时 answer_id 非空，
而 action 值为 "show" 或者 "skip" 时 answer_id 为空；
q_num 表示当前会话中问题的编号。

输入：
+------+-----------+--------------+------------+-----------+------------+
| id   | action    | question_id  | answer_id  | q_num     | timestamp  |
+------+-----------+--------------+------------+-----------+------------+
| 5    | show      | 285          | null       | 1         | 123        |
| 5    | answer    | 285          | 124124     | 1         | 124        |
| 5    | show      | 369          | null       | 2         | 125        |
| 5    | skip      | 369          | null       | 2         | 126        |
+------+-----------+--------------+------------+-----------+------------+
输出：
+-------------+
| survey_log  |
+-------------+
|    285      |
+-------------+
解释：
问题 285 的回答率为 1/1，而问题 369 回答率为 0/1，因此输出 285 。

# Write your MySQL query statement below
select question_id survey_log
from survey_log
group by question_id
having sum(answer_id is not null)/count(*) =
(
    select sum(answer_id is not null)/count(*) prect
    from survey_log
    group by question_id
    order by prect desc limit 1
)