create table test (col_a number,col_b number);
insert into test values(1,1);
insert into test values(1,2);
insert into test values(1,3);
insert into test values(2,4);
insert into test values(2,5);
select AA.*
  from test AA,
       (select col_a, max(col_b) col_b
          from test
         where col_a in
               (select col_a
                  from (select col_a, count(distinct col_b)
                          from test
                         group by col_a
                        having count(distinct col_b) > 1) temp)
         group by col_a) tt
 where AA.col_a = tt.col_a
   and AA.col_b < tt.col_b;

------解决方案--------------------
看来又理解错了，楼主是要删除重复记录。总是被你的删除重复列给弄晕了，如下：
SQL code

Delete From T Where (a, b) NOT IN (Select A, max(B) as B From T Group By A)