root@localhost : test 02:18:41>select * from us;
+-------+------+-------+---------------------+
| rowid | user | score | testtime            |
+-------+------+-------+---------------------+
|     1 | a    |    80 | 2012-09-03 14:11:31 |
|     2 | a    |    60 | 2012-09-03 14:11:53 |
|     3 | a    |    75 | 2012-09-03 14:11:57 |
|     4 | b    |    59 | 2012-09-03 14:12:14 |
|     5 | b    |    69 | 2012-09-03 14:12:22 |
|     6 | b    |    79 | 2012-09-03 14:12:42 |
|     7 | c    |    90 | 2012-09-03 14:12:57 |
|     8 | c    |    95 | 2012-09-03 14:13:04 |
|     9 | d    |    85 | 2012-09-03 14:13:13 |
+-------+------+-------+---------------------+
9 rows in set (0.00 sec)

root@localhost : test 02:18:46>select a.* from us a where 2>(select count(*) from us b where a.user=b.user and  b.testtime > a.testtime );
+-------+------+-------+---------------------+
| rowid | user | score | testtime            |
+-------+------+-------+---------------------+
|     2 | a    |    60 | 2012-09-03 14:11:53 |
|     3 | a    |    75 | 2012-09-03 14:11:57 |
|     5 | b    |    69 | 2012-09-03 14:12:22 |
|     6 | b    |    79 | 2012-09-03 14:12:42 |
|     7 | c    |    90 | 2012-09-03 14:12:57 |
|     8 | c    |    95 | 2012-09-03 14:13:04 |
|     9 | d    |    85 | 2012-09-03 14:13:13 |
+-------+------+-------+---------------------+

------解决方案--------------------
分组取前两条吧  
SQL code

SELECT a.rowid,a.user,a.score,a.testtime 
FROM score_us a 
LEFT JOIN score_us b 
ON a.user = b.user AND a.testtime < b.testtime 
GROUP BY a.rowid,a.user,a.score,a.testtime 
HAVING COUNT(b.rowid) < 2 
ORDER BY a.testtime

------解决方案--------------------
root@localhost : test 02:18:46>select a.* from us a where 2>(select count(*) from us b where a.user=b.user and  b.testtime > a.testtime );  
  +


这个可以