Incorrect order result for sum(column) of group by

[yanhuqing666]

mysql> select c_flag ,sum(c_decimal) x from sharding_four_node a group by c_flag order by x;
+--------+---------+
| c_flag | x       |
+--------+---------+
| 1      | 1.0000  |
| 10     | 10.0000 |
| 11     | 11.0000 |
| 12     | 12.0000 |
| 2      | 2.0000  |
| 3      | 3.0000  |
| 4      | 4.0000  |
| 5      | 5.0000  |
| 6      | 6.0000  |
| 7      | 7.0000  |
| 8      | 8.0000  |
| 9      | 9.0000  |
+--------+---------+