Leetcode1113.报告的记录（简单）

题目
动作表：Actions

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user_id       | int     |
| post_id       | int     |
| action_date   | date    | 
| action        | enum    |
| extra         | varchar |
+---------------+---------+

此表没有主键，所以可能会有重复的行。
action 字段是 ENUM 类型的，包含:('view', 'like', 'reaction', 'comment', 'report', 'share')
extra 字段是可选的信息（可能为 null），其中的信息例如有：1.报告理由(a reason for report) 2.反应类型(a type of reaction)

编写一条SQL，查询每种 报告理由（report reason）在昨天的报告数量。假设今天是 2019-07-05。

查询及结果的格式示例：

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       | 4       | 2019-07-04  | view   | null   |
| 2       | 4       | 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       | 2019-07-04  | view   | null   |
| 5       | 2       | 2019-07-04  | report | racism |
| 5       | 5       | 2019-07-04  | view   | null   |
| 5       | 5       | 2019-07-04  | report | racism |
+---------+---------+-------------+--------+--------+

Result table:

+---------------+--------------+
| report_reason | report_count |
+---------------+--------------+
| spam          | 1            |
| racism        | 2            |
+---------------+--------------+ 

注意，我们只关心报告数量非零的结果。

产生数据

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', '4', '2019-07-04', 'view', 'None');
INSERT INTO Actions (user_id, post_id, action_date, ACTION, extra) VALUES ('2', '4', '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');

解答
选出时间为前一天 且action为report 的记录

SELECT * 
FROM Actions AS A
WHERE DATEDIFF('2019-07-05', A.`action_date`) = 1 AND A.`action` = 'report';

对extra分组统计post_id的数量

SELECT A.`extra` AS report_reason, COUNT(DISTINCT A.`post_id`) AS report_count
FROM Actions AS A
WHERE DATEDIFF('2019-07-05', A.`action_date`) = 1 AND A.`action` = 'report'
GROUP BY A.`extra`
ORDER BY A.`extra` DESC;