use tempdb;
go

create table dbo.t1 (id int primary key, a int);
create table dbo.t2 (id int identity primary key, t1_id int not null, b int);

insert into dbo.t1 values (1, 1);

insert into dbo.t2
 (t1_id, b)
 select top (2000000)
  1, 2
 from
  master.dbo.spt_values a cross join
  master.dbo.spt_values b;
go

set statistics time, io on;
go

select
 t1.*
from
 dbo.t1 t1 left join
 dbo.t2 t2 on t2.t1_id = t1.id
where
 t2.t1_id is null
option
 (maxdop 1);

select
 t1.*
from
 dbo.t1 t1 
where
 not exists(select 1 from dbo.t2 t2 where t2.t1_id = t1.id)
option
 (maxdop 1);
go

set statistics time, io off;
go

drop table dbo.t1, dbo.t2;
go

(0 rows affected)
Table 't2'. Scan count 1, logical reads 5215, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 't1'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 172 ms,  elapsed time = 174 ms.

(0 rows affected)
Table 't2'. Scan count 1, logical reads 3, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 't1'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 0 ms.