Db2 11.1

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

with MYTABLE (ID, SEQ, DATE_FROM, DATE_TO, NOTES) as (
  values
  ('ELEM1', 1, date '2022-04-01', date '2023-03-31', 'First part'),
  ('ELEM1', 2, date '2022-04-01', date '2023-03-31', 'Second part'),
  ('ELEM1', 3, date '2022-04-01', date '2023-03-31', 'Third part'),
  ('ELEM2', 1, date '2018-11-12', date '2099-12-31', 'First part'),
  ('ELEM2', 2, date '2018-11-12', date '2099-12-31', 'Second part'),
  ('ELEM2', 3, date '2018-11-12', date '2099-12-31', 'Third part'),
  ('ELEM2', 4, date '2018-11-12', date '2099-12-31', 'Forth part')
),
allnotes (ID, seq, ALLNOTES) as (
  select ID, seq, varchar(notes, 4000) from MYTABLE where seq = 1
  union all
  select mytable.id, mytable.seq, allnotes.allnotes || ' ' || mytable.notes from allnotes, mytable where (mytable.id, mytable.seq) = (allnotes.id, allnotes.seq + 1)
),
groups as (
  select id, min(date_from) date_from, max(date_to) date_to, max(seq) seq from mytable group by id
)
select groups.id, date_from, date_to, allnotes from groups inner join allnotes on (allnotes.id, allnotes.seq) = (groups.id, groups.seq)

ID	DATE_FROM	DATE_TO	ALLNOTES
ELEM1	2022-04-01	2023-03-31	First part Second part Third part
ELEM2	2018-11-12	2099-12-31	First part Second part Third part Forth part