SQL & PL/SQL

Hi all.

Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi

PL/SQL Release 10.2.0.4.0 - Production                         

CORE 10.2.0.4.0 Production                                       

TNS for Linux: Version 10.2.0.4.0 - Production                 

NLSRTL Version 10.2.0.4.0 - Production

I am working with NOT IN and comparing against multiple columns, I know that when there is a NULL value returned by the sub-select the entire set is evaluated to false but in this case when I have tow columns and only the second column is null I get the rows returned, for example:

select 'true' from dual where (1,2) not in (SELECT null,2 FROM dual);
no rows selected
select 'true' from dual where (1,2) not in (SELECT 2,null FROM dual);
'TRU
----
true

Could somebody please explain to me why this happen?

Thanks in advance, regards.