declare @table1 table ( id1 int, val1 varchar(8))
insert into @table1 
select 1, 'a' union all
select 2, 'b' union all
select 3, 'c' union all
select 4, 'd' union all
select 5, 'e' 

declare @table2 table ( id2 int, val2 varchar(8))
insert into @table2
select 1, 'a' union all
select 2, 'b' union all
select 3, 'c' 

solution1 : select t1.val1
from @table1 t1 
left join @table2 t2 on t2.val2 = t1.val1
where t2.val2 is null 

solution2 : select t1.val1
from @table1 t1 
where not exists ( select val2 from @table2 where val2 = t1.val1 )