SQL Server 2012 and NULL comparison -

can explain me why these 2 statements returns different results?

select case when not((null = null) or (1 != 1)) 1 else 0 end select case when not((null = null) , (1 != 1)) 1 else 0 end 

i know null compared gives false , wanted use property stopped @ commands similar above. real statements instead of nulls use variables can null simplified them show problem. thought has operation order seems that's not it.

i know null compared gives false

this isn't correct, null compared evaluates unknown, not false, quick example:

select case when (null = null) 'true'              when not(null = null) 'false'             else 'other'         end 

will give third option of other.

if rewrite logic (still same meaning, becomes more clear):

select case when (null <> null) , (1 = 1) 1 else 0 end select case when (null <> null) or (1 = 1) 1 else 0 end 

so in first instance have when [unknown] , [true] false, in second have when [unknown] or [true] true, returns 1.

if rewrite query variables, inspect execution plan xml, can see sql server rewrites expression above during compilation:

declare @a int = null, @b int = null, @c int = 1, @d int = 1;  select top 1          case when not((@a = @b) or (@c != @d)) 1 else 0 end,         case when not((@a = @b) , (@c != @d)) 1 else 0 end