Unexpected results from SELECT FROM WHERE X NOT IN Y - sql

Question

So...
select COUNT(*) cnt from docDocument d
inner join tblAttachment a on d.AttachmentID = a.ID
where
a.ContainerID = 1
Returns 6673
select COUNT(*) from tblAttachment
where
ContainerID = 1
Returns 10372
select COUNT(*) cnt from docDocument d
right join tblAttachment a on d.AttachmentID = a.ID
where
a.ContainerID = 1
AND
d.ID IS NULL
Returns 3699 which makes sense as 10372 - 6673 = 3699
SELECT COUNT(*) FROM
(
select ID from tblAttachment a
where
a.ContainerID = 1
Except
(
SELECT AttachmentId from docDocument
)
) tst
Unsurprisingly returns 3699... However...
select COUNT(*) from tblAttachment a
where
a.ContainerID = 1 AND
a.ID NOT IN
(
SELECT d.AttachmentId from docDocument d
)
I was expecting this to return 3699 but surprisingly it's returning 0.
Can anyone explain these results?

Answer 1

If the subquery returns a null value, the NOT IN is no longer true, and no rows are returned.
Either aviod null values to be returned:
select COUNT(*) from tblAttachment a
where
a.ContainerID = 1 AND
a.ID NOT IN
(
SELECT d.AttachmentId from docDocument d WHERE d.AttachmentId IS NOT NULL
)
Or switch to the "null safe" NOT EXISTS:
select COUNT(*) from tblAttachment a
where
a.ContainerID = 1 AND
NOT EXISTS
(
SELECT * from docDocument d WHERE d.AttachmentId = a.ID
)

Answer 2

Your not in query return nulls value. So its displaying zero records.
This is the below scenario you are getting right now. Let's try to understand that and make the changes.
select *
into #tab1
from (select 'a' Name union all
select 'b'union all
select 'c'union all
select 'd'union all
select 'e') AS A
select *
into #tab2
from (select 'd' Name union all
select 'e' union all
select NULL) AS A
Not Exists
select Name
from #tab1
where not exists (select *
from #tab2
where #tab1.Name = #tab2.Name )
Left Join
select t1.Name
from #tab1 t1
left join #tab2 t2 on t1.Name = t2.Name
where t2.Name is null
Not in
select *
from #tab1
where Name not in (select Name from #tab2)
Note: whenever a Not in query returns NULL, the outer query returns blank data.

Answer 3

You must be having NULL values in docDocument table...That's why you are not getting anything.
Please try like this USE NOT EXISTS
select COUNT(*) from tblAttachment a
where
a.ContainerID = 1 AND
NOT EXISTS
(
SELECT d.AttachmentId from docDocument d
WHERE a.ID = d.ID
)
You can use EXCEPT operator.
select * into #tab1 from (
select 'a' Name union all
select 'b'union all
select 'c'union all
select 'd'union all
select 'e'
)AS A
select * into #tab2 from
(
select 'd' Name union all
select 'e' union all
select NULL
)AS A
SOLUTION
select Name from #tab1
EXCEPT
select * from #tab2
OUTPUT
Name
----
a
b
c
(3 rows affected)