MySQL中提供了很多函数，为我们的SQL操作提供便利，例如：

    2. mysql> select  NOW(), DATE_FORMAT( NOW(),'%Y-%m-%d %H:%i:%s') as time;
    3. +---------------------+---------------------+
    4. | NOW()               | time                |
    5. +---------------------+---------------------+
    6. | 2021-10-14 14:28:09 | 2021-10-14 14:28:09 |
    7. +---------------------+---------------------+
    1. mysql> select  NOW(), DATE_FORMAT( NOW(),'%Y-%m-%d') as time;
    2. +---------------------+------------+
    3. | NOW()               | time       |
    4. +---------------------+------------+
    5. | 2021-10-14 14:28:58 | 2021-10-14 |
    6. +---------------------+------------+

    更多函数：https://dev.mysql.com/doc/refman/5.7/en/functions.html