Leetcode1141.查询近30天活跃用户数（简单）

题目
活动纪录表：Activity

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user_id       | int     |
| session_id    | int     |
| activity_date | date    |
| activity_type | enum    |
+---------------+---------+

该表是用户在社交网站的活动记录。
该表没有主键，可能包含重复数据。
activity_type 字段为以下四种值 ('open_session', 'end_session', 'scroll_down', 'send_message')。
每个 session_id 只属于一个用户。

请写SQL查询出截至 2019-07-27（包含2019-07-27），近 30天的每日活跃用户（当天只要有一条活跃记录，即为活跃用户），

查询结果示例如下：

Activity table:

+---------+------------+---------------+---------------+
| user_id | session_id | activity_date | activity_type |
+---------+------------+---------------+---------------+
| 1       | 1          | 2019-07-20    | open_session  |
| 1       | 1          | 2019-07-20    | scroll_down   |
| 1       | 1          | 2019-07-20    | end_session   |
| 2       | 4          | 2019-07-20    | open_session  |
| 2       | 4          | 2019-07-21    | send_message  |
| 2       | 4          | 2019-07-21    | end_session   |
| 3       | 2          | 2019-07-21    | open_session  |
| 3       | 2          | 2019-07-21    | send_message  |
| 3       | 2          | 2019-07-21    | end_session   |
| 4       | 3          | 2019-06-25    | open_session  |
| 4       | 3          | 2019-06-25    | end_session   |
+---------+------------+---------------+---------------+

Result table:

+------------+--------------+ 
| day        | active_users |
+------------+--------------+ 
| 2019-07-20 | 2            |
| 2019-07-21 | 2            |
+------------+--------------+ 

非活跃用户的记录不需要展示。

生成数据

CREATE TABLE Activity1 (user_id INT, session_id INT, activity_date DATE, activity_type VARCHAR(20));
 
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('1', '1', '2019-07-20', 'open_session');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('1', '1', '2019-07-20', 'scroll_down');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('1', '1', '2019-07-20', 'end_session');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('2', '4', '2019-07-20', 'open_session');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('2', '4', '2019-07-21', 'send_message');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('2', '4', '2019-07-21', 'end_session');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('3', '2', '2019-07-21', 'open_session');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('3', '2', '2019-07-21', 'send_message');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('3', '2', '2019-07-21', 'end_session');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('4', '3', '2019-06-25', 'open_session');
INSERT INTO Activity1 (user_id, session_id, activity_date, activity_type) VALUES ('4', '3', '2019-06-25', 'end_session');

解答
首先选择近 30天的每日活跃用户

SELECT *
FROM Activity1 AS A
WHERE DATEDIFF('2019-07-27', A.`activity_date`) <= 30;

然后对日期进行分组 选出每个组的uid个数(去重)

SELECT A.`activity_date` AS DAY, COUNT(DISTINCT A.`user_id`) AS active_users
FROM Activity1 AS A
WHERE DATEDIFF('2019-07-27', A.`activity_date`) <= 30
GROUP BY  A.`activity_date`;