Recursive CTE

with a(code, num) as( 
  select 'A', 1 from dual union all 
  select 'B', 5 from dual union all 
  select 'C', 12 from dual union all 
  select 'D', 14 from dual 
) 
, b(code, next_num, val) as ( 
  select 
    a.code 
    , lead(num - 1, 1, num) over(order by code asc) as next_num
    , num
  from a
  
  union all
  
  select
    code
    , next_num
    , val + 1
  from b
  where val < next_num
) 
select 
  b.code 
  , val
from b
order by 2 asc

with a(code, num) as( 
  select 'A', 1 from dual union all 
  select 'B', 5 from dual union all 
  select 'C', 12 from dual union all 
  select 'D', 14 from dual 
) 
, b as ( 
  select 
    a.* 
    , lead(num - 1, 1, num) over(order by code asc) as next_num 
  from a 
) 
select 
  b.code 
  , gen.val 
from b 
  cross join lateral( 
    select num + level - 1 as val 
    from dual 
    connect by num + level - 1 <= next_num 
  ) gen
order by 2 asc