Oracle 11g Release 2

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CREATE TABLE X
    (id int, section int)
;
insert into X 
select 1, 6 from dual
union all select 2, 6 from dual
union all select 3, 7 from dual
union all select 4, 7 from dual
union all select 5, 6 from dual;
5 rows affected

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;


SECTION
SECTION_NR
FIRST_ID


6
3
5

7
2
3

6
1
1