（1）将时间转换成字符串
将系统时间转换成字符串
mysql> select date_format(now(), '%y%m%d %h:%i:%s');
+---------------------------------------+
| date_format(now(), '%y%m%d %h:%i:%s') |
+---------------------------------------+
| 160510 06:10:30 |
+---------------------------------------+
扩展：
%Y：年
%c：月
%d：日
%H：小时
%i：分钟
%s：秒partition_key字段信息为：160101
mysql> select date_format(partition_key,'%y-%m-%d') from t_order ;
+---------------------------------------+
| date_format(partition_key,'%y-%m-%d') |
+---------------------------------------+
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
| 16-01-01 |
+---------------------------------------+
14 rows in set (1.67 sec)
