如果表的ID 不是自增的：

root@localhost : test 10:00:21>select * from tb_test;
+------+---------------------+--------+--------+
| id   | time                | value1 | value2 |
+------+---------------------+--------+--------+
|    1 | 2012-09-01 09:41:00 |   33.4 |   56.7 |
+------+---------------------+--------+--------+
1 row in set (0.00 sec)

root@localhost : test 09:59:37>delimiter $$
root@localhost : test 10:00:17>create trigger insert_tb
    -> before insert on tb_test
    -> for each row
    -> begin
    -> select max(value1) into @a from tb_test;
    -> if new.value1 < @a then set new.value1= @a;
    -> end if;
    -> end $$
Query OK, 0 rows affected (0.07 sec)

root@localhost : test 10:00:36>insert into tb_test values(1,now(),'12.4','56.7');
Query OK, 1 row affected (0.03 sec)

root@localhost : test 10:00:51>select * from tb_test;
+------+---------------------+--------+--------+
| id   | time                | value1 | value2 |
+------+---------------------+--------+--------+
|    1 | 2012-09-01 09:41:00 |   33.4 |   56.7 |
|    1 | 2012-09-01 10:00:51 |   33.4 |   56.7 |
+------+---------------------+--------+--------+
2 rows in set (0.00 sec)

root@localhost : test 10:00:54>insert into tb_test values(1,now(),'55.4','56.7');
Query OK, 1 row affected (0.00 sec)

root@localhost : test 10:01:03>select * from tb_test;
+------+---------------------+--------+--------+
| id   | time                | value1 | value2 |
+------+---------------------+--------+--------+
|    1 | 2012-09-01 09:41:00 |   33.4 |   56.7 |
|    1 | 2012-09-01 10:00:51 |   33.4 |   56.7 |
|    1 | 2012-09-01 10:01:03 |   55.4 |   56.7 |
+------+---------------------+--------+--------+
3 rows in set (0.00 sec)


是自增的话，修改下赋值这行就可以了。