use tempdb;
set ansi_nulls, quoted_identifier, xact_abort on;
go

drop table if exists dbo.N;
create table dbo.N (n int primary key);
insert into dbo.N
 (n)
 select top (2000000)
  row_number() over (order by 1/0)
 from
  master.dbo.spt_values a cross join
  master.dbo.spt_values b;
go

declare @v int, @n int = 1000000;

set statistics time on;

select @v = n from dbo.N where n <= @n;

;With a as (
Select t.n from (values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9)) t(n)
)
Select top(@n)
@v = a0.n + isNull(a1.n,0)*10 + isNull(a2.n,0)*100 + isNull(a3.n,0)*1000 + isNull(a4.n,0)*10000 + isNull(a5.n,0)*100000
from a a0
cross join a a1
cross join a a2
cross join a a3
cross join a a4
cross join a a5

;With a as (
Select t.n from (values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9)) t(n)
)
Select top(@n)
@v = row_number() over (order by 1/0)
from a a0
cross join a a1
cross join a a2
cross join a a3
cross join a a4
cross join a a5

select @v = row_number() over (order by (select 1/0))
from string_split(REPLICATE(cast(' ' as varchar(max)), @n-1), ' ') x

set statistics time off;

(2000000 rows affected)

 SQL Server Execution Times:
   CPU time = 94 ms,  elapsed time = 93 ms.

 SQL Server Execution Times:
   CPU time = 141 ms,  elapsed time = 146 ms.

 SQL Server Execution Times:
   CPU time = 109 ms,  elapsed time = 96 ms.

 SQL Server Execution Times:
   CPU time = 3672 ms,  elapsed time = 3693 ms.