>Now I want to delete all cars that *has* a record in "drivers", but the driver_name is null
>
>so far, I always do
>
>delete from cars
>where
>(
> select drivers.driver_name
> from drivers
> where
> cars.id_drivers = drivers.id
> )
> is null
>
>that works fine, but I just wander if there is a better way... just for curiosity...

What is the "best" way depends on several factors, including whether cars or "null" drivers has the higest number of records and which Firebird version you use. I would expect that in many cases, the most efficient query would be similar to

EXECUTE BLOCK as
declare i int;
begin
for select d.id
from drivers d
where d.driver_name is null
into :i
do
begin
delete from cars c
where c.id_drivers = :i;
end
end

(I rarely use EXECUTE BLOCK, so there are probably some spelling mistakes)

Execute block only works with Firebird 2+, the more traditional way to write your delete would either be to use a stored procedure or:

delete from cars c
where
exists(select *
from drivers d
where c.id_drivers = d.id
and d.driver_name is null)

HTH,
Set