Um Schlüsselwerte in SQL Server vorab zu generieren, verwenden Sie ein Sequenz statt einer IDENTITY-Spalte.
zB
drop table if exists t
drop table if exists #t_stg
drop sequence t_seq
go
create sequence t_seq start with 1 increment by 1
create table t(id int primary key default (next value for t_seq),a int, b int)
create table #t_stg(id int, a int, b int)
insert into #t_stg(a,b) values (1,2),(3,3),(4,5)
update #t_stg set id = next value for t_seq
--select * from #t_stg
insert into t(id,a,b)
select * from #t_stg
