SQL Server 2017

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create table main (id int, "date" int);
create table old (id int, "date" int);
create trigger old_to_old on main instead of insert as begin
insert into old (id, "date") select id, "date" from inserted where "date" < 5;
insert into main (id, "date") select id, "date" from inserted where "date" >= 5;
end;
insert into main values (1,1), (4,4), (7,7), (10,10)
8 rows affected

select *, 'main' from_where from main
union all
select *, 'old' from old


id
date
from_where


7
7
main

10
10
main

1
1
old

4
4
old