You are looking at something like this:

declare @Table1 table(

Col1 char(1),

Col2 int

);

declare @Table2 table(

Col1 char(1)

);

insert into @Table1

values ('A',1),

('B',1),

('C',1),

('A',2),

('B',2),

('C',2),

('D',2);

insert into @Table2

values ('A'),('B'),('C'),('D');

select * from @Table1;

select * from @Table2;

with UniqCol2 as (

select distinct Col2 from @Table1

), ExcludedData as (

select

t2.Col1,

uc.Col2

from

@Table2 t2

cross apply UniqCol2 uc

except

select

t1.Col1,

t1.Col2

from

@Table1 t1)

select Col1 from ExcludedData;