求教一条SQL语句的写法
有这样的一个表：
Time                Sign                   SN
2013-01-01     AAA                   1
2013-01-04     CCC                   2
2013-01-06     BBB                    3
2013-01-05     BBB                    1
2013-01-02     AAA                   2
2013-01-05     BBB                    2
2013-01-06     BBB                    4
2013-01-03     AAA                   3
2013-01-04     AAA                   4
2013-01-03     CCC                   1
排序后要求是这样的：
Sign                Time                   SN
AAA                2013-01-01       1
AAA                2013-01-02       2
AAA                2013-01-03       3
AAA                2013-01-04       4
CCC                2013-01-03       1
CCC                2013-01-04       2
BBB                 2013-01-05       1
BBB                 2013-01-05       2
BBB                 2013-01-06       3
BBB                 2013-01-06       4