# Game Play Analysis IV Problem

## Description

LeetCode Problem 550.

Table: Activity

``````1
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+--------------+---------+
| Column Name  | Type    |
+--------------+---------+
| player_id    | int     |
| device_id    | int     |
| event_date   | date    |
| games_played | int     |
+--------------+---------+
(player_id, event_date) is the primary key of this table.
This table shows the activity of players of some game.
Each row is a record of a player who logged in and played a number of games (possibly 0) before logging out on some day using some device.
``````

Write an SQL query that reports the fraction of players that logged in again on the day after the day they first logged in, rounded to 2 decimal places. In other words, you need to count the number of players that logged in for at least two consecutive days starting from their first login date, then divide that number by the total number of players.

The query result format is in the following example:

``````1
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Activity table:
+-----------+-----------+------------+--------------+
| player_id | device_id | event_date | games_played |
+-----------+-----------+------------+--------------+
| 1         | 2         | 2016-03-01 | 5            |
| 1         | 2         | 2016-03-02 | 6            |
| 2         | 3         | 2017-06-25 | 1            |
| 3         | 1         | 2016-03-02 | 0            |
| 3         | 4         | 2018-07-03 | 5            |
+-----------+-----------+------------+--------------+

Result table:
+-----------+
| fraction  |
+-----------+
| 0.33      |
+-----------+
Only the player with id 1 logged back in after the first day he had logged in so the answer is 1/3 = 0.33
``````

## MySQL Solution

``````1
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select round(a_frac.playerCount / count(distinct a_full.player_id), 2) as fraction
from Activity a_full,
(select count(distinct a1.player_id) as playerCount
from Activity a1
inner join
(select player_id, min(event_date) as first_login
from Activity
group by player_id) a2
on a1.player_id = a2.player_id and datediff(a1.event_date, a2.first_login) = 1) a_frac
``````

The third select statement gets all the first login dates. The second select statement gets players who login the day after their first login dates. The first select statement calculates the fraction.