dense_rank是你想要的。
样本数据：

SQL> with temp (part, bom) as
  2    (select 'part1', 'bom1' from dual union all
  3     select 'part1', 'bom1' from dual union all
  4     select 'part1', 'bom2' from dual union all
  5     select 'part1', 'bom2' from dual union all
  6     select 'part1', 'bom3' from dual union all
  7     select 'part1', 'bom4' from dual union all
  8     --
  9     select 'part2', 'bom12' from dual union all
 10     select 'part2', 'bom13' from dual union all
 11     select 'part2', 'bom14' from dual union all
 12     select 'part2', 'bom15' from dual union all
 13     select 'part2', 'bom16' from dual union all
 14     select 'part2', 'bom17' from dual union all
 15     --
 16     select 'part3', 'bom14' from dual union all
 17     --
 18     select 'part4', 'bom14' from dual
 19    )

字符串
查询：

20  select part, bom,
 21    dense_rank() over (partition by part order by bom) seq
 22  from temp;

PART  BOM          SEQ
----- ----- ----------
part1 bom1           1
part1 bom1           1
part1 bom2           2
part1 bom2           2
part1 bom3           3
part1 bom4           4
part2 bom12          1
part2 bom13          2
part2 bom14          3
part2 bom15          4
part2 bom16          5
part2 bom17          6
part3 bom14          1
part4 bom14          1

14 rows selected.

SQL>

型