Table: Activity
+--------------+---------+
| Column Name | Type |
+--------------+---------+
| player_id | int |
| device_id | int |
| event_date | date |
| games_played | int |
+--------------+---------+
(player_id, event_date) 是這個表的兩個主鍵
這個表顯示的是某些遊戲玩家的遊戲活動情況
每一行是在某天使用某個設備登出之前登錄並玩多個遊戲（可能爲0）的玩家的記錄
請編寫一個 SQL 查詢，描述每一個玩家首次登陸的設備名稱
查詢結果格式在以下示例中：
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 |
+-----------+-----------+
#方法1
select a.player_id,a.device_id from Activity a where (a.player_id,a.event_date) in(select #player_id,min(event_date) as 'first_login' from Activity group by player_id)
#方法2
SELECT a.player_id,a.device_id FROM Activity a INNER JOIN (SELECT player_id,MIN(event_date) AS 'latest_date' FROM Activity GROUP BY player_id) AS b ON a.event_date = b.latest_date AND a.player_id = b.player_id;
#方法3
SELECT a.player_id,a.device_id FROM Activity a INNER JOIN (SELECT player_id,MIN(event_date) AS 'latest_date' FROM Activity GROUP BY player_id) AS b ON a.event_date = b.latest_date
where a.player_id = b.player_id;
寫一條 SQL 查詢語句獲取每位玩家 第一次登陸平臺的日期。
select player_id, min(event_date) as `first_login`
from Activity
group by player_id
order by player_id asc
查詢結果的格式如下所示：
Result 表：
+-----------+-------------+
| player_id | first_login |
+-----------+-------------+
| 1 | 2016-03-01 |
| 2 | 2017-06-25 |
| 3 | 2016-03-02 |
+-----------+-------------+