2995. Viewers Turned Streamers

Description

Table: Sessions

+---------------+----------+
| Column Name   | Type     |
+---------------+----------+
| user_id       | int      |
| session_start | datetime |
| session_end   | datetime |
| session_id    | int      |
| session_type  | enum     |
+---------------+----------+
session_id is column of unique values for this table.
session_type is an ENUM (category) type of (Viewer, Streamer).
This table contains user id, session start, session end, session id and session type.

Write a solution to find the number of streaming sessions for users whose first session was as a viewer.

Return the result table ordered by count of streaming sessions, user_id in descending order.

The result format is in the following example.

Example 1:

Input: 
Sessions table:
+---------+---------------------+---------------------+------------+--------------+
| user_id | session_start       | session_end         | session_id | session_type | 
+---------+---------------------+---------------------+------------+--------------+
| 101     | 2023-11-06 13:53:42 | 2023-11-06 14:05:42 | 375        | Viewer       |  
| 101     | 2023-11-22 16:45:21 | 2023-11-22 20:39:21 | 594        | Streamer     |   
| 102     | 2023-11-16 13:23:09 | 2023-11-16 16:10:09 | 777        | Streamer     | 
| 102     | 2023-11-17 13:23:09 | 2023-11-17 16:10:09 | 778        | Streamer     | 
| 101     | 2023-11-20 07:16:06 | 2023-11-20 08:33:06 | 315        | Streamer     | 
| 104     | 2023-11-27 03:10:49 | 2023-11-27 03:30:49 | 797        | Viewer       | 
| 103     | 2023-11-27 03:10:49 | 2023-11-27 03:30:49 | 798        | Streamer     |  
+---------+---------------------+---------------------+------------+--------------+
Output: 
+---------+----------------+
| user_id | sessions_count | 
+---------+----------------+
| 101     | 2              | 
+---------+----------------+
Explanation
- user_id 101, initiated their initial session as a viewer on 2023-11-06 at 13:53:42, followed by two subsequent sessions as a Streamer, the count will be 2.
- user_id 102, although there are two sessions, the initial session was as a Streamer, so this user will be excluded.
- user_id 103 participated in only one session, which was as a Streamer, hence, it won't be considered.
- User_id 104 commenced their first session as a viewer but didn't have any subsequent sessions, therefore, they won't be included in the final count. 
Output table is ordered by sessions count and user_id in descending order.

Solutions

Solution 1: Window Function + Equi-Join

We can use the window function RANK() to rank each session by user_id dimension, and record it in table T. Then, we equi-join T and the Sessions table by user_id, and filter out the records in T where the rank is 1, and session_type is Viewer, and session_type in the Sessions table is Streamer. Finally, we group by user_id and sum up.

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# Write your MySQL query statement below
WITH
    T AS (
        SELECT
            user_id,
            session_type,
            RANK() OVER (
                PARTITION BY user_id
                ORDER BY session_start
            ) AS rk
        FROM Sessions
    )
SELECT user_id, COUNT(1) AS sessions_count
FROM
    T AS t
    JOIN Sessions AS s USING (user_id)
WHERE rk = 1 AND t.session_type = 'Viewer' AND s.session_type = 'Streamer'
GROUP BY 1
ORDER BY 2 DESC, 1 DESC;