1142. User Activity for the Past 30 Days II

Description

Table: Activity

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user_id       | int     |
| session_id    | int     |
| activity_date | date    |
| activity_type | enum    |
+---------------+---------+
This table may have duplicate rows.
The activity_type column is an ENUM (category) of type ('open_session', 'end_session', 'scroll_down', 'send_message').
The table shows the user activities for a social media website. 
Note that each session belongs to exactly one user.

 

Write a solution to find the average number of sessions per user for a period of 30 days ending 2019-07-27 inclusively, rounded to 2 decimal places. The sessions we want to count for a user are those with at least one activity in that time period.

The result format is in the following example.

 

Example 1:

Input: 
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   |
| 3       | 5          | 2019-07-21    | open_session  |
| 3       | 5          | 2019-07-21    | scroll_down   |
| 3       | 5          | 2019-07-21    | end_session   |
| 4       | 3          | 2019-06-25    | open_session  |
| 4       | 3          | 2019-06-25    | end_session   |
+---------+------------+---------------+---------------+
Output: 
+---------------------------+ 
| average_sessions_per_user |
+---------------------------+ 
| 1.33                      |
+---------------------------+
Explanation: User 1 and 2 each had 1 session in the past 30 days while user 3 had 2 sessions so the average is (1 + 1 + 2) / 3 = 1.33.

Solutions

Solution 1

SQL Code
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# Write your MySQL query statement below
WITH
    T AS (
        SELECT
            COUNT(DISTINCT session_id) AS sessions
        FROM Activity
        WHERE activity_date <= '2019-07-27' AND DATEDIFF('2019-07-27', activity_date) < 30
        GROUP BY user_id
    )
SELECT IFNULL(ROUND(AVG(sessions), 2), 0) AS average_sessions_per_user
FROM T;

Solution 2

SQL Code
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SELECT
    IFNULL(
        ROUND(COUNT(DISTINCT session_id) / COUNT(DISTINCT user_id), 2),
        0
    ) AS average_sessions_per_user
FROM Activity
WHERE DATEDIFF('2019-07-27', activity_date) < 30;