SQL Server 2017

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create table test(Id INT, Field1 char(1), Field2 varchar(max));

insert into test (id, Field1) values (1, 'a');
insert into test (id, Field1) values (2, 'b');
insert into test (id, Field1) values (3, 'c');
insert into test (id, Field1) values (4, 'd');
4 rows affected

select t.*, t2.new_field2
from test t cross apply
     (select string_agg(concat(id, field1), '') within group (order by id) as new_field2
      from test t2
      where t2.id <= t.id
     ) t2;


Id
Field1
Field2
new_field2


1
a
null
1a

2
b
null
1a2b

3
c
null
1a2b3c

4
d
null
1a2b3c4d


with toupdate as (
      select t.*, t2.new_field2
      from test t cross apply
           (select string_agg(concat(id, field1), '') within group (order by id) as new_field2
            from test t2
            where t2.id <= t.id
           ) t2
     )
update toupdate
    set field2 = new_field2;
4 rows affected

select *
from test


Id
Field1
Field2


1
a
1a

2
b
1a2b

3
c
1a2b3c

4
d
1a2b3c4d