SQL Server 2019

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create table set1
(
  day int
);

insert into set1 (day) values (1),(2),(3),(1),(2),(3),(4); -- contains 2x 3

select * from set1;

create table set2
(
  day int
);

insert into set2 (day) values (1),(2),(1),(2),(4); -- removed 3's

select * from set1;

day

1

2

3

1

2

3

4

day

1

2

3

1

2

3

4

select set1.day
from set1
  except
select set2.day
from set2;

--> only unique values remain...

day

3

select set1.day
from set1
where not exists ( select 'x'
                   from set2
                   where set2.day = set1.day );

day

3

3

select set1.day
from set1
left join set2
  on set2.day = set1.day
where set2.day is null;

day

3

3