This is what I did:
create table a (a int, a1 char(8));
create table b (b int);
insert into a values (1, 'one');
insert into a values (NULL, 'null');
and this is what I got from Informix:
> select * from a where a not in (select * from b);
a a1
1 one
null
2 row(s) retrieved.
> select * from a where not (a in (select * from b));
a a1
1 one
null
2 row(s) retrieved.
> select * from a where not a in (select * from b);
a a1
1 one
null
2 row(s) retrieved.
> select * from a where a<>(select * from b);
a a1
No rows found.
> select * from a where a=(select * from b);
a a1
No rows found.
Andreas