Testing for inequality in T-SQL

Question

I\'ve just come across this in a WHERE clause:

AND NOT (t.id = @id)

How does this compare with:

AND t.id != @id
         


        

Answer 1

Just a little adjustement fors those who come later:

The equality operator generate a unknow value when there is a null and the unknown value is treated a false. Not (unknown) is unknown

In the example below I'll try to say if a couple (a1, b1) is equal to (a2, b2). Note that each columns has 3 values 0, 1 and NULL.

DECLARE @t table (a1 bit, a2 bit, b1 bit, b2 bit)

Insert into @t (a1 , a2, b1, b2) 
values( 0 , 0 , 0 , NULL )

select 
a1,a2,b1,b2,
case when (
    (a1=a2 or (a1 is null and a2 is null))
and (b1=b2 or (b1 is null and b2 is null))
)
then 
'Equal'
end,
case when not (
    (a1=a2 or (a1 is null and a2 is null))
and (b1=b2 or (b1 is null and b2 is null))
)
then 
'not Equal'
end,
case when (
    (a1<>a2 or (a1 is null and a2 is not null) or (a1 is not null and a2 is null))
or (b1<>b2 or (b1 is null and b2 is not null) or (b1 is not null and b2 is null))
)
then 
'Different'
end
from @t

Note that here we expect results :

• Equal to be null
• not equal to be not equal
• different to be difFerent

but we get another result

• Equal is null OK
• Not Equal is null ???
• Different is different