树形结构排序

要实现的功能如下：
有一个表，存有id和父id，已知层次最多不超过四层，
ID NAME PARENT_ID
10 AAAA -1
20 BBBB -1
30 CCCC -1
101 DDDD 10
102 EEEE 10
103 FFFF 10
1011 GGGG 101
1012 HHHH 101
1021 IIII 102
201 JJJJ 20
2011 KKKK 201
...

求一查询sql，得到如下结果：
ID NAME
10 AAAA
101 DDDD
1011 GGGG
1012 HHHH
102 EEEE
1021 IIII
103 FFFF
20 BBBB
201 JJJJ
2011 KKKK
30 CCCC

就是得到一个树形结构的顺序。

SQL code

mysql> SELECT * FROM TTL1;
+------+------+-----------+
| ID   | NAME | PARENT_ID |
+------+------+-----------+
|   10 | AAAA |        -1 |
|   20 | BBBB |        -1 |
|   30 | CCCC |        -1 |
|  101 | DDDD |        10 |
|  102 | EEEE |        10 |
|  103 | FFFF |        10 |
| 1011 | GGGG |       101 |
| 1012 | HHHH |       101 |
| 1021 | IIII |       102 |
|  201 | JJJJ |        20 |
| 2011 | KKKK |       201 |
+------+------+-----------+
11 rows in set (0.00 sec)

mysql> SELECT * FROM ttl1 a ORDER BY LEFT(CONCAT(a.id,'0000'),4);
+------+------+-----------+
| ID   | NAME | PARENT_ID |
+------+------+-----------+
|   10 | AAAA |        -1 |
|  101 | DDDD |        10 |
| 1011 | GGGG |       101 |
| 1012 | HHHH |       101 |
|  102 | EEEE |        10 |
| 1021 | IIII |       102 |
|  103 | FFFF |        10 |
|   20 | BBBB |        -1 |
|  201 | JJJJ |        20 |
| 2011 | KKKK |       201 |
|   30 | CCCC |        -1 |
+------+------+-----------+
11 rows in set (0.00 sec)

mysql>

select * from <table> order by id; 试试

很专业啊