game-play-analysis-ii.sql

-- 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 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 a SQL query that reports the device that is first logged in for each player.

-- The query result format is in the following example:

-- Activity table:
-- +-----------+-----------+------------+--------------+
-- | player_id | device_id | event_date | games_played |
-- +-----------+-----------+------------+--------------+
-- | 1         | 2         | 2016-03-01 | 5            |
-- | 1         | 2         | 2016-05-02 | 6            |
-- | 2         | 3         | 2017-06-25 | 1            |
-- | 3         | 1         | 2016-03-02 | 0            |
-- | 3         | 4         | 2018-07-03 | 5            |
-- +-----------+-----------+------------+--------------+

-- Result table:
-- +-----------+-----------+
-- | player_id | device_id |
-- +-----------+-----------+
-- | 1         | 2         |
-- | 2         | 3         |
-- | 3         | 1         |
-- +-----------+-----------+

-- V0 
SELECT a.player_id, 
       a.device_id 
FROM   activity a 
WHERE ( a.player_id, a.event_date) in 
(SELECT 
player_id, 
min(event_date) 
FROM 
activity
GROUP BY 1 )

-- V1 

-- V2 
-- Time:  O(n)
-- Space: O(n)
SELECT player_id, 
       device_id 
FROM   activity 
WHERE  ( player_id, event_date ) IN (SELECT player_id, 
                                            Min(event_date) 
                                     FROM   activity 
                                     GROUP  BY player_id 
                                     ORDER  BY NULL)