Grouping of PARTITION BY / GROUP BY only until next section to obtain a list of sections

I have a table like this:

id	section
1	6
2	6
3	7
4	7
5	6

and would like to obtain a grouped list that says

section	section_nr	first_id
6	1	1
7	2	3
6	3	5

Using ROW_NUMBER twice I am able to obtain something close:

SELECT section, ROW_NUMBER() OVER (ORDER BY id) AS section_nr, id as first_id
FROM (
  SELECT id, section, ROW_NUMBER() OVER (PARTITION BY section ORDER BY id) AS nr_within
  FROM X
)
WHERE nr_within = 1

section	section_nr	first_id
6	1	1
7	2	3

… but of course the second section 6 is missing, since PARTITION BY groups all section=6 together. Is it somehow possible to only group until the next section?

More generally (regarding GROUP BY instead of PARTITION BY), is there a simple solution to group (1,1,2,2,1) to (1,2,1) instead of (1,2)?

Answer

This is a typical gaps and islands problem that can be solved like this:

with u as
(select id, section, 
case when section = lag(section) over(order by id) then 0 else 1 end as grp
from X),
v as
(select id, 
section, 
sum(grp) over(order by id) as section_nr
from u)
select section,
section_nr,
min(id) as first_id
from v
group by section, section_nr;

Basically you keep tabs in a column where there is a change in section by comparing current section to section from the row above (ordered by id). Whenever there is a change, set this column to 1, when no change set it to 0. The rolling sum of this column will be the section number. Getting first_id is a simple matter of using group by.

Fiddle