mysql order by null

mysql> explain select * from user group by name,age \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
table: user
         type: ALL
possible_keys: NULL
key: NULL
      key_len: NULL
          ref: NULL
         rows: 36
        Extra: Using temporary; Using filesort
1 row in set (0.00 sec)

mysql> explain select * from user group by name,age order by null \G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
table: user
         type: ALL
possible_keys: NULL
key: NULL
      key_len: NULL
          ref: NULL
         rows: 36
        Extra: Using temporary
1 row in set (0.00 sec)

注意有下划线的红色字体，是没有加‘order by null’出现的提示，表示进行了排序，以下是结果：

mysql> select * from user group by name,age order by null
+-------------+------+----+
| name        | age  | id |
+-------------+------+----+
| zhangzhibin |   13 |  1 |
| lisi        |   13 |  2 |
| wangwu      |   18 |  3 |
| zhangzhibin |   19 |  4 |
| lisi        |   12 |  5 |
| wangwu      |   13 |  6 |
| zhangzhibin |   11 |  7 |
| lisi        |   14 |  8 |
| wangwu      |   16 |  9 |
| zhangzhibin |   18 | 10 |
| wangwu      |   11 | 12 |
| lisi        |   15 | 14 |
| wangwu      |   10 | 15 |
| lisi        |   18 | 17 |
| wangwu      |   17 | 18 |
| zhangzhibin |   12 | 19 |
| lisi        |   10 | 20 |
| lisi        |   19 | 23 |
| wangwu      |   14 | 24 |
| lisi        |   11 | 26 |
| zhangzhibin |   17 | 28 |
| wangwu      |   15 | 30 |
| zhangzhibin |   14 | 34 |
+-------------+------+----+
23 rows in set (0.00 sec)

mysql> select * from user group by name,age;
+-------------+------+----+
| name        | age  | id |
+-------------+------+----+
| lisi        |   10 | 20 |
| lisi        |   11 | 26 |
| lisi        |   12 |  5 |
| lisi        |   13 |  2 |
| lisi        |   14 |  8 |
| lisi        |   15 | 14 |
| lisi        |   18 | 17 |
| lisi        |   19 | 23 |
| wangwu      |   10 | 15 |
| wangwu      |   11 | 12 |
| wangwu      |   13 |  6 |
| wangwu      |   14 | 24 |
| wangwu      |   15 | 30 |
| wangwu      |   16 |  9 |
| wangwu      |   17 | 18 |
| wangwu      |   18 |  3 |
| zhangzhibin |   11 |  7 |
| zhangzhibin |   12 | 19 |
| zhangzhibin |   13 |  1 |
| zhangzhibin |   14 | 34 |
| zhangzhibin |   17 | 28 |
| zhangzhibin |   18 | 10 |
| zhangzhibin |   19 |  4 |
+-------------+------+----+
23 rows in set (0.00 sec)

为了显示出order by null 这句的作用我选择了 name 和 age 两个字段进行分组,若只选name来进行分组，那么最后的结果只有三条，不容易看出结果