MySQLでdatetime型の演算をする

以下のようなテーブルがある．

MariaDB [staging]> desc guests;
+---------------+----------------------+------+-----+---------+----------------+
| Field         | Type                 | Null | Key | Default | Extra          |
+---------------+----------------------+------+-----+---------+----------------+
| id            | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| department_id | smallint(5) unsigned | YES  | MUL | NULL    |                |
| user_id       | varchar(10)          | YES  |     | NULL    |                |
| comment       | varchar(255)         | YES  |     | NULL    |                |
| entered_at    | datetime             | NO   |     | NULL    |                |
| updated_at    | datetime             | NO   |     | NULL    |                |
| exited_at     | datetime             | YES  |     | NULL    |                |
+---------------+----------------------+------+-----+---------+----------------+
7 rows in set (0.036 sec)

現状では，JSTでdatetime型のカラムに記録されている．

MariaDB [staging]> select * from guests;
+----+---------------+----------+-----------------------------+---------------------+---------------------+---------------------+
| id | department_id | user_id  | comment                     | entered_at          | updated_at          | exited_at           |
+----+---------------+----------+-----------------------------+---------------------+---------------------+---------------------+
|  3 |             1 | C0117123 | これはテストです〜          | 2019-08-08 02:41:20 | 2019-08-08 20:41:20 | 2020-03-02 23:00:09 |

ここでは entered_at カラムの時刻をUTCへ修正する．

UPDATE guests SET entered_at=entered_at - INTERVAL 9 HOUR

修正されたことが確認できる．

MariaDB [staging]> select * from guests;
+----+---------------+----------+-----------------------------+---------------------+---------------------+---------------------+
| id | department_id | user_id  | comment                     | entered_at          | updated_at          | exited_at           |
+----+---------------+----------+-----------------------------+---------------------+---------------------+---------------------+
|  3 |             1 | C0117123 | これはテストです〜          | 2019-08-07 17:41:20 | 2019-08-08 20:41:20 | 2020-03-02 23:00:09 |