力扣1132(MySQL)-报告的记录Ⅱ（中等）

题目：

编写一段 SQL 来查找：在被报告为垃圾广告的帖子中，被移除的帖子的每日平均占比，四舍五入到小数点后 2 位。

Actions 表：

Removals 表：

Result 表：

2019-07-04 的垃圾广告移除率是 50%，因为有两张帖子被报告为垃圾广告，但只有一个得到移除。
2019-07-02 的垃圾广告移除率是 100%，因为有一张帖子被举报为垃圾广告并得到移除。
其余几天没有收到垃圾广告的举报，因此平均值为：(50 + 100) / 2 = 75%
注意，输出仅需要一个平均值即可，我们并不关注移除操作的日期。

建表语句：

 1 Create table If Not Exists actions_1132 (
 2     user_id int,
 3         post_id int,
 4         action_date date,
 5         action enum('view', 'like', 'reaction', 'comment', 'report', 'share'),
 6         extra varchar(6)
 7 );
 8 Create table If Not Exists removals_1132 (
 9         post_id int,
10         remove_date date
11     );
12 Truncate table actions_1132;
13 insert into actions_1132 (user_id, post_id, action_date, action, extra) values
14 ('1', '1', '2019-07-01','view',null),
15 ('1', '1', '2019-07-01','like',null),
16 ('1', '1', '2019-07-01','share',null),
17 ('2', '2', '2019-07-04','view',null),
18 ('2', '2', '2019-07-04','report','spam'),
19 ('3', '4', '2019-07-04','view',null),
20 ('3', '4', '2019-07-04','report','spam'),
21 ('4', '3', '2019-07-02','view',null),
22 ('4', '3', '2019-07-02','report','spam'),
23 ('5', '2', '2019-07-03','view',null),
24 ('5', '2', '2019-07-03','report','racism'),
25 ('5', '5', '2019-07-03','view',null),
26 ('5', '5', '2019-07-03','report','racism');
27 Truncate table removals_1132;
28 insert into removals_1132 (post_id, remove_date) values ('2', '2019-07-20'),( '3', '2019-07-18');

解题思路：

①先筛选出actions表中被报道为垃圾信息即extra = 'spam'的数据；

1 select post_id, action_date
2 from actions_1132
3 where extra = 'spam'

②将第一步查询出来的表通过post_id与 removals左连接；

1 select *
2 from (
3     select post_id, action_date
4     from actions_1132
5     where extra = 'spam'
6   ) as a
7 left join removals_1132 b
8 on a.post_id = b.post_id

③然后再根据第二步查询出的临时表，以action_date为分组，统计出删除比率；

1   select action_date,count(distinct b.post_id) / count(distinct a.post_id) as rate
2   from (
3     select post_id, action_date
4     from actions_1132
5     where extra = 'spam'
6   ) as a
7   left join removals_1132 b
8   on a.post_id = b.post_id
9   group by action_date

④最后根据上一步的临时表，计算出被移除的帖子的每日平均占比；

 1 select round(sum(rate)/ count(*)*100, 2) as average_daily_percent
 2 from (
 3   select action_date,count(distinct b.post_id) / count(distinct a.post_id) as rate
 4   from (
 5     select post_id, action_date
 6     from actions_1132
 7     where extra = 'spam'
 8   ) as a
 9   left join removals_1132 b
10   on a.post_id = b.post_id
11   group by action_date
12 ) as temp