Postgres 12

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create table bestellungen4  (ID int,   sellingnr int,   rgnr  int, correction_rg boolean);
insert into bestellungen4
values
(1, 1, 0,  NULL),
(2, 2, 1,  NULL),
(3, 3, 2,  NULL),
(4, 4,  NULL,  NULL),
(5, 2,  NULL,  NULL),
(6, 3,  NULL,  NULL);
6 rows affected

update bestellungen4
  set correction_rg = true
where id in (select id 
             from (
               select id, 
                      row_number() over (partition by sellingnr order by id) as rn
               from bestellungen4
             ) t
             where rn > 1);
2 rows affected

select *
from bestellungen4
order by id;


id
sellingnr
rgnr
correction_rg


1
1
0
null

2
2
1
null

3
3
2
null

4
4
null
null

5
2
null
t

6
3
null
t

id	sellingnr	rgnr	correction_rg
1	1	0	null
2	2	1	null
3	3	2	null
4	4	null	null
5	2	null	t
6	3	null	t