Oracle - Convert value from rows into ranges

Question

Are there any techniques that would allow a row set like this

WITH 
base AS
(
    SELECT  1 N FROM DUAL UNION ALL
    SELECT  2 N FROM DUAL UNION ALL
    SEL         


        

Answer 1

Yet another way:

with base as (
    select  1 n from dual union all
    select  2 n from dual union all
    select  3 n from dual union all
    select  6 n from dual union all
    select  7 n from dual union all
    select 17 n from dual union all
    select 18 n from dual union all
    select 19 n from dual union all
    select 21 n from dual)
select a,b 
from (select a
            ,case when b is not null and a is not null
                  then b
                  else lead(n) over (order by n)
            end b
      from (select n
                  ,a
                  ,b
            from (select n
                        ,case n-1 when lag (n) over (order by n) then null else n end a
                        ,case n+1 when lead (n) over (order by n) then null else n end b
                  from base)
      where a is not null
         or b is not null))
where a is not null
order by a