task: #550

Author: Ivanbyone

README.md

@@ -80,6 +80,7 @@ Have a good contributing!
 2. [Medium](./leetcode/medium/)
     - [176. Second Highest Salary](./leetcode/medium/176.%20Second%20Highest%20Salary.sql)
     - [184. Department Highest Salary](./leetcode/medium/184.%20Department%20Highest%20Salary.sql)
+    - [550. Game Play Analysis IV](./leetcode/medium/550.%20Game%20Play%20Analysis%20IV.sql)
     - [570. Managers with at Least 5 Direct Reports](./leetcode/medium/570.%20Managers%20with%20at%20Least%205%20Direct%20Reports.sql)
     - [602. Friend Requests II: Who Has the Most Friends](./leetcode/medium/602.%20Friend%20Requests%20II:%20Who%20Has%20the%20Most%20Friends.sql)
     - [626. Exchange Seats](./leetcode/medium/626.%20Exchange%20Seats.sql)

leetcode/medium/550. Game Play Analysis IV.sql

@@ -0,0 +1,41 @@
+/*
+Question 550. Game Play Analysis IV
+Link: https://leetcode.com/problems/game-play-analysis-iv/description/?envType=study-plan-v2&envId=top-sql-50
+
+Table: Activity
+
++--------------+---------+
+| Column Name  | Type    |
++--------------+---------+
+| player_id    | int     |
+| device_id    | int     |
+| event_date   | date    |
+| games_played | int     |
++--------------+---------+
+(player_id, event_date) is the primary key (combination of columns with unique values) of this table.
+This table shows the activity of players of some games.
+Each row is a record of a player who logged in and played a number of games (possibly 0) before logging out on someday using some device.
+
+Write a solution to report 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 determine the number of players who logged in on the day immediately following their initial login, and divide it by the number of total players.
+*/
+
+WITH logged_again AS (
+    SELECT 
+        player_id,
+        MIN(event_date) + INTERVAL '1 day' AS next_event_date
+    FROM Activity
+    GROUP BY player_id
+)
+
+SELECT
+    ROUND(COUNT(CASE
+        WHEN la.next_event_date IS NOT NULL
+            THEN 1
+    END) / COUNT(DISTINCT a.player_id)::NUMERIC, 2) AS fraction
+FROM Activity AS a
+LEFT JOIN
+    logged_again AS la
+    ON
+        a.event_date = la.next_event_date
+        AND a.player_id = la.player_id
+