Oracle下这样的记录如何排重？

有一张表，字段和记录如下：
SID         TID       NAME       DEPT       STEP     TIME
1             253       ABC         MIS         5           10:20
2             253       ABC         MIS         6           10:22
3             254       STD         PE           5           11:20
3             254       STD         PE           6           11:30

现在需要SELECT出253和254最后时间(TIME)的纪录，请问如何写SQL?

------解决方案--------------------
SQL> create table t(
2 sid number,
3 tid number,
4 name varchar2(20),
5 dept varchar2(30),
6 step number,
7 ttime varchar2(20)
8 );

Table created

Executed in 1.391 seconds

SQL> insert into t select 1,253, 'ABC ', 'MIS ',5, '10:20 ' FROM DUAL
2 UNION ALL SELECT 2,253, 'ABC ', 'MIS ',6, '10:22 ' FROM DUAL
3 UNION ALL SELECT 3,254, 'STD ', 'PE ',5, '10:22 ' FROM DUAL
4 UNION ALL SELECT 4,254, 'STD ', 'PE ',6, '10:30 ' FROM DUAL;

4 rows inserted

Executed in 0.297 seconds

SQL> SELECT * FROM T;

SID TID NAME DEPT STEP TTIME
---------- ---------- -------------------- ------------------------------ ---------- --------------------
1 253 ABC MIS 5 10:20
2 253 ABC MIS 6 10:22
3 254 STD PE 5 10:22
4 254 STD PE 6 10:30

Executed in 0.187 seconds

SQL> SELECT SID,TID,NAME,DEPT,STEP,TTIME FROM (
2 SELECT SID,TID,NAME,DEPT,STEP,TTIME,ROW_NUMBER()OVER(PARTITION BY TID ORDER BY TTIME DESC) RM
3 FROM T
4 ) A WHERE A.RM = 1;

SID TID NAME DEPT STEP TTIME
---------- ---------- -------------------- ------------------------------ ---------- --------------------
2 253 ABC MIS 6 10:22
4 254 STD PE 6 10:30

Executed in 0.172 seconds