mysql的一个日期问题，理解不了
mysql> select date_sub('2014-03-04',interval dayofmonth(curdate())-1 day);
+-------------------------------------------------------------+
| date_sub('2014-03-04',interval dayofmonth(curdate())-1 day) |
+-------------------------------------------------------------+
| 2014-02-14                                                  |
+-------------------------------------------------------------+
1 row in set (0.00 sec)

把这个日期插入到表中：
mysql> select * from t_date_3;
+---------------------+------+
| t_create            | val  |
+---------------------+------+
| 2014-03-17 14:08:13 |    6 |
| 2014-03-17 14:08:31 |   11 |
| 2014-03-04 00:00:00 | NULL |
+---------------------+------+
3 rows in set (0.00 sec)

然后：
mysql> select date_sub(t_create,interval dayofmonth(t_create)-1 day) from t_date
_3;
+--------------------------------------------------------+
| date_sub(t_create,interval dayofmonth(t_create)-1 day) |
+--------------------------------------------------------+
| 2014-03-01 14:08:13                                    |
| 2014-03-01 14:08:31                                    |
| 2014-03-01 00:00:00                                    |
+--------------------------------------------------------+
3 rows in set (0.00 sec)

注意红色部分，谁给解释一下？