select l, s, row_number() over(partition by s order by l), case when row_number() over(partition by s order by l) = 1 then s end
from (select level l, case when level<=5 then 'A' else 'B' end s from dual connect by level <= 10)
 
         L S ROW_NUMBER()OVER(PARTITIONBYSO CASEWHENROW_NUMBER()OVER(PARTI
---------- - ------------------------------ ------------------------------
         1 A                              1 A
         2 A                              2 
         3 A                              3 
         4 A                              4 
         5 A                              5 
         6 B                              1 B
         7 B                              2 
         8 B                              3 
         9 B                              4 
        10 B                              5