> 6- A comparison expression involving one operator where one operand is null
> evaluates always to false.

More precisely, it evaluates to null.
(It can be important if you combine more conditions together).
When you use it in IF statement, then the command following THEN clause
is executed if the condition is TRUE, otherwise you continue after ELSE
(if the condition is FALSE or NULL).




btw. did you try this command:
select count(*), count(data_consegna_seggio)
from v_avanzamento_pratica
where
( datagara between :tdal and :tal ) and
( ditta_aggiudicataria <> '' )
into :quante_so, :consegnate_seggio;


Ivan
http://www.volny.cz/iprenosil/interbase