subselect and left join not working?

Hi,

I am using postgres 8.4 and I am having problems doing a very simple query:

select zona_id from zonas where zona_id not in (select zona_id from usuarios
where per_id =2)

but I get no records in return.

The problem is that, if I I break down the quey, I get:

select zona_id from zonas

"A"
"B"
"C"
"D"
"E"
"F"
"FCHIH"
"FCOAH"
"DGO"
"T"
"FGTO"
"FGRO"
"FQRO"
"FHGO"
"JAL"
"MOR"
"FPUE"
"FQROO"
"FSLP"
"F VER"
"FYUC"
""

select zona_id from usuarios where per_id =2

"E"
"F"
"FCHIH"
"JAL"
"FPUE"
"F VER"
"C"
"D"
"A"
""

but with

select zona_id from zonas where zona_id not in (select zona_id from usuarios
where per_id =2)

I dont' get records when I am expecting to see this:

"B"
"FCOAH"
"DGO"
"T"
"FGTO"
"FGRO"
"FQRO"
"FHGO"
"MOR"
"FQROO"
"FSLP"
"FYUC"
""

What's wrong? Please help. Any clues?

---
It also does not work a left join on 'zonas'. Some 'zonas' are designated to
some users but in my case is not wroking:

t1
usr zone
1 A
2 C
3 D

t2
zone
A
B
C
D
E

I want to see
zone usr
A 1
B
C 2
D 3
E