日付の切り捨て・切り上げ

DATE()を使う方法

 mysql> SELECT DATE(NOW());
 +-------------+
 | DATE(NOW()) |
 +-------------+
 | 2019-06-07  |
 +-------------+

DATE_FORMAT()を使う方法

 mysql> SELECT DATE_FORMAT(NOW(), ' %Y-%m-01');
 +--------------------------------+
 | DATE_FORMAT(NOW(), '%Y-%m-01') |
 +--------------------------------+
 | 2019-06-01                     |
 +--------------------------------+

参考

• https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_date-format