mysql> select * from test;
+------+------+------+
| id   | name | num1 |
+------+------+------+
|    1 | a    |    1 |
|    2 | b    |    2 |
|    3 | c    |    3 |
|    5 | e    |    5 |
|    6 | f    |    6 |
+------+------+------+
5 rows in set (0.00 sec)

mysql> update test set num1=(select t.rownum from(
    -> select a.id,a.name,count(*) as rownum from test a,test b
    -> where a.num1>=b.num1 group by a.id,a.name) t where test.id=t.id);
Query OK, 2 rows affected (0.02 sec)
Rows matched: 5  Changed: 2  Warnings: 0

mysql> select * from test;
+------+------+------+
| id   | name | num1 |
+------+------+------+
|    1 | a    |    1 |
|    2 | b    |    2 |
|    3 | c    |    3 |
|    5 | e    |    4 |
|    6 | f    |    5 |
+------+------+------+
5 rows in set (0.00 sec)

mysql>

------解决方案--------------------
如果你要重新对ID进行编号，只有导入到一张新的自增表中