题目
动作表： Actions
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| user_id | int |
| post_id | int |
| action_date | date |
| action | enum |
| extra | varchar |
+---------------+---------+
这张表没有主键，并有可能存在重复的行。
action 列的类型是 ENUM，可能的值为 ('view', 'like', 'reaction', 'comment', 'report', 'share')。
extra 列拥有一些可选信息，例如：报告理由（a reason for report）或反应类型（a type of reaction）等。
移除表： Removals
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| post_id | int |
| remove_date | date |
+---------------+---------+
这张表的主键是 post_id。
这张表的每一行表示一个被移除的帖子，原因可能是由于被举报或被管理员审查。
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。
查询结果的格式如下：
Actions table:
+---------+---------+-------------+--------+--------+
| user_id | post_id | action_date | action | extra |
+---------+---------+-------------+--------+--------+
| 1 | 1 | 2019-07-01 | view | null |
| 1 | 1 | 2019-07-01 | like | null |
| 1 | 1 | 2019-07-01 | share | null |
| 2 | 2 | 2019-07-04 | view | null |
| 2 | 2 | 2019-07-04 | report | spam |
| 3 | 4 | 2019-07-04 | view | null |
| 3 | 4 | 2019-07-04 | report | spam |
| 4 | 3 | 2019-07-02 | view | null |
| 4 | 3 | 2019-07-02 | report | spam |
| 5 | 2 | 2019-07-03 | view | null |
| 5 | 2 | 2019-07-03 | report | racism |
| 5 | 5 | 2019-07-03 | view | null |
| 5 | 5 | 2019-07-03 | report | racism |
+---------+---------+-------------+--------+--------+
Removals table:
+---------+-------------+
| post_id | remove_date |
+---------+-------------+
| 2 | 2019-07-20 |
| 3 | 2019-07-18 |
+---------+-------------+
Result table:
+-----------------------+
| average_daily_percent |
+-----------------------+
| 75.00 |
+-----------------------+
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。
生成数据
DROP TABLE Actions;
CREATE TABLE Actions (user_id INT, post_id INT, action_date DATE, ACTION VARCHAR(20), extra VARCHAR(20));
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('1', '1', '2019-07-01', 'view', 'None');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('1', '1', '2019-07-01', 'like', 'None');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('1', '1', '2019-07-01', 'share', 'None');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('2', '2', '2019-07-04', 'view', 'None');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('2', '2', '2019-07-04', 'report', 'spam');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('3', '4', '2019-07-04', 'view', 'None');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('3', '4', '2019-07-04', 'report', 'spam');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('4', '3', '2019-07-02', 'view', 'None');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('4', '3', '2019-07-02', 'report', 'spam');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('5', '2', '2019-07-04', 'view', 'None');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('5', '2', '2019-07-04', 'report', 'racism');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('5', '5', '2019-07-04', 'view', 'None');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('5', '5', '2019-07-04', 'report', 'racism');
CREATE TABLE Removals(
post_id INT,
remove_date DATE );
INSERT INTO Removals VALUE(2, '2019-07-20'), (3, '2019-07-18');
解答
题目要求的结果是：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。
在被报告为垃圾广告的帖子为Actions表中extra为spam的记录 但其不一定都出现在Removals表中
进而可以求得移除帖子的每日占比
最后求得 移除帖子的每日平均占比
解答
先对两边进行连接
SELECT *
FROM Actions a
LEFT JOIN Removals b
ON a.post_id = b.post_id;
选出被报告为垃圾广告的帖子
SELECT *
FROM Actions a
LEFT JOIN Removals b
ON a.post_id = b.post_id
WHERE a.extra = 'spam';
对日期进行分组 统计a.post_id的数量即为垃圾邮件数量 统计b.post_id即为删除的数量
SELECT a.`action_date`, COUNT(DISTINCT a.`post_id`) AS spamCount, COUNT(DISTINCT b.`post_id`) AS delCount
FROM Actions a
LEFT JOIN Removals b
ON a.post_id = b.post_id
WHERE a.extra = 'spam'
GROUP BY a.`action_date`;
delCount/spamCount即可得到求得移除帖子的每日占比
SELECT a.`action_date`, COUNT(DISTINCT b.`post_id`)/COUNT(DISTINCT a.`post_id`) AS daily_percent
FROM Actions a
LEFT JOIN Removals b
ON a.post_id = b.post_id
WHERE a.extra = 'spam'
GROUP BY a.`action_date`;
子查询对每日占比取平均即为每日平均占比
SELECT ROUND(AVG(tmp.daily_percent),2) AS average_daily_percent
FROM (SELECT a.`action_date`, COUNT(DISTINCT b.`post_id`)/COUNT(DISTINCT a.`post_id`)*100 AS daily_percent
FROM Actions a
LEFT JOIN Removals b
ON a.post_id = b.post_id
WHERE a.extra = 'spam'
GROUP BY a.`action_date`) AS tmp;
别人的解答
SELECT round(SUM(delCount / spamCount * 100) / COUNT(DISTINCT action_date), 2) AS average_daily_percent
FROM (
-- count行数
SELECT action_date, COUNT(distinct a.post_id) AS spamCount, count(distinct b.post_id) AS delCount
FROM Actions a
LEFT JOIN Removals b ON a.post_id = b.post_id
where a.extra = 'spam'
GROUP BY a.action_date
) a
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