Problem

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 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 an SQL query to report the device that is first logged in for each player.

Return the result table in any order.

Examples

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         |
+-----------+-----------+

Solution

Method 1 - Subquery

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);

Method 2 - Window Function

SELECT player_id, device_id FROM
(
	SELECT player_id, device_id, event_date, 
	MIN(event_date) OVER (PARTITION BY player_id ORDER BY event_idate) as first login
	FROM Activity;
) as t1
WHERE t1.event_date = t1.first_login