SQLite 3.27

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CREATE TABLE Messages (`id` INTEGER, `sender_id` INTEGER, `message` VARCHAR(4), `date` TEXT);
INSERT INTO Messages (`id`, `sender_id`, `message`, `date`) VALUES
  ('1', '1', 'Cya', '2020-10-10'),
  ('2', '2', 'Bye', '2020-10-10'),
  ('3', '1', 'Heya', '2020-10-11');
select id, sender_id, message, date, type 
from (
  select id, sender_id, message, date, 'message' as type, 2 sort
  from Messages
  union all
  select lag(max(id), 1, min(id)) over (order by date), null, null, date, 'date', 1
  from Messages
  group by date
  union all
  select * from (
    select id, null, null, date, 'date', 3
    from Messages
    order by date desc, id desc limit 1
  )  
)
order by date, sort, id


id
sender_id
message
date
type


1
null
null
2020-10-10
date

1
1
Cya
2020-10-10
message

2
2
Bye
2020-10-10
message

2
null
null
2020-10-11
date

3
1
Heya
2020-10-11
message

3
null
null
2020-10-11
date

id	sender_id	message	date	type
1	null	null	2020-10-10	date
1	1	Cya	2020-10-10	message
2	2	Bye	2020-10-10	message
2	null	null	2020-10-11	date
3	1	Heya	2020-10-11	message
3	null	null	2020-10-11	date