Oracle - Convert value from rows into ranges - sql

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
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.N
FROM base a
to yield results
1 3
6 7
17 19
21 21
It is in effect a rows to ranges operation.
I'm playing in Oracle Land, and would appreciate any suggestions.

Answer 1

I feel like this can probably be improved on, but it works:
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
)
, lagged AS
(
SELECT n, LAG(n) OVER (ORDER BY n) lag_n FROM base
)
, groups AS
(
SELECT n, row_number() OVER (ORDER BY n) groupnum
FROM lagged
WHERE lag_n IS NULL OR lag_n < n-1
)
, grouped AS
(
SELECT n, (SELECT MAX(groupnum) FROM groups
WHERE groups.n <= base.n
) groupnum
FROM base
)
SELECT groupnum, MIN(n), MAX(n)
FROM grouped
GROUP BY groupnum
ORDER BY groupnum

Answer 2

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 min(n), max(n) from
(
select n, connect_by_root n root from base
connect by prior n = n-1
start with n not in (select n from base b
where exists (select 1 from base b1 where b1.n = b.n-1)
)
)
group by root
order by root

Answer 3

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