IN Operator SQL - sql

Question

I have a table called NUMS with a single column n.
And I fill values 1,2,3,4,5,null in it.
Now a query
SELECT n FROM Nums
WHERE n IN (1, 2, null)
In this case I guess it's converted to
SELECT n FROM Nums
Where n = 1 OR n = 2 OR n = null
I am also comparing n with a null value which should yield unknown and it should return an empty set.But it's returning 1,2 (null is not returning, although included in IN operator)
Now a query
SELECT n FROM Nums WHERE n NOT IN(1, 2, null)
...gets converted to:
SELECT n FROM Nums
Where n!=1 AND n!=2 AND n!=null
Here what I said above works and it does not return anything.
Can anyone explain in detail what's happening.

Answer 1

This is because null = null is always false the operator to use for null is IS or IS NOT
You can use the query below for the expected output
SELECT n FROM Nums WHERE n IN (1,2) OR n IS NULL
[Edit]Thanx #Buckwad

Answer 2

OK I have found the answer
SELECT n FROM Nums
WHERE n NOT IN (1, 2, null)
evaluates to
SELECT n FROM Nums
n!=1 AND n!=2 AND n!=null
The outcome of last comparison will always be UNKNOWN.
and the truth table of AND shows that as soon as one Unknown is invloved in it (U,T)(U,F),(U,U) the reult can only be U or F (U=Unknown, F=False) and hence it will not be included in the result set.
In case of
SELECT n FROM Nums
WHERE n IN (1, 2, null)
equates to
SELECT n FROM Nums
WHERE n = 1 OR n =2 OR n=null
Now for the row with n=1, the operation n=1 will come as true
and for the row with n=2, the operation n=2 will come as true
and for all rows n=null will be unknown
So it gives 1 and 2 in the result set.
Hope u people liked it.
CAN ANYONE PLEASE MARK MY REPLY AS ANSWER

Answer 3

null is not returning, although included in IN operator
Because of the n = NULL evaluation; NULL can't equal NULL. It would need to be handled as n IS NULL (or similar appropriate syntax) to return the NULL row/record.

Answer 4

You cannot compare directly with null. To find whether the value of a column is null, you must use something along the lines of this:
SELECT n
FROM Nums
WHERE n IS NULL

Answer 5

I am also comparing n with a null value which should yield unknown and it should return an empty set.
it shouldn't
if you need in additional row filled with NULL values the best way imo is:
SELECT n FROM Nums WHERE n IN(1,2)
UNION
SELECT NULL
Now a query
SELECT n FROM Nums WHERE n NOT IN(1, 2, null)
...gets converted to:
Here what I said above works and it does not return anything.
thats right, because nothing can be equal to NULL and for NULL comparison IS NULL statement is used

Answer 6

If an IN list contains a NULL value anywhere, the result of the entire list is UNKNOWN
With the correct default of ANSI NULLS, when you compare NULL with NULL the result is not true, but UNKNOWN

Answer 7

SELECT n FROM Nums
WHERE n IN (1, 2)
Or n Is null
This will retrieve what you intend to get. As said by Mitch normally a comparison to Null yields UNKNOWN. This is because NULL itself is defined as an undefined value. It's like saying I lived in Nottingham, Birmingham and somewhere. Finding somewhere on the world map can prove a bit tricky as it is undefined.