Can anyone explain Diff between except and Not in.?

Question

Hi experts,
Sometimes "not in" function is not working.Can anyone tell me the reason.please
Query 1:-
Select POL from table_1 where pol not in (select pol from table_2)
Query 2:-
select pol from table_1 except select pol from table_2
Getting output from Query 2. Query1 dosen't displaying output.
Thanks and Regards

Answer 1

While using Except
the resultant result is given after removing duplicates
the resultant result shall include NULL values as well as present in the source table
While using NOT IN
the resultant result is AS IS from the source table (no duplicate elimination)
But, it DOES ignore the NULL values.
The following code should explain in a better way!
declare @tab1 table ( id int)
insert @tab1 select 1
insert @tab1 select 1
insert @tab1 select 2
insert @tab1 select 2
insert @tab1 select 3
insert @tab1 select NULL
declare @tab2 table ( id int)
insert @tab2 select 2
insert @tab2 select 2
--using NOT IN
--duplicates returned/NULL ignored
select id from @tab1
where id not in (select id from @tab2)
--using NOT EXISTS
--duplicates returned/NULL also returned
select id from @tab1 t1
where not exists(select id from @tab2 t2 where t1.id=t2.id)
--using Except
--duplicates Eliminated/NULL returned
select id from @tab1
EXCEPT
select id from @tab2
Thanks,
Jay
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