subquery, except and view

Hi,

Please, I need help to correct some sql queries...

Let me define my db :

CLIENT (NUM_CLT, NAME,   ADR)
ITEM   (NUM_IT,  NAME,   PRICE)
COMMAND(NUM_CLT, NUM_IT, QT)

I meet two problems : one concerns subqueries or
operator instruction "except" and the other, the view.

I don't understand why the two queries below
are not correct with Postgresql though it be correct with Oracle
(I want to know the clients which have not 
made more than 2 commands) :

------- 
QUERY 1
-------

select clt.num_clt 
from client clt
where clt.num_clt not in  (select cd.num_clt    from commande cd   group by cd.num_clt   having count(*) > 2);

The PostgreSQL Error is : rewrite: aggregate column view must be at rigth side in qual

I test the subquery and it is ok.

------- 
QUERY 2
------- 

select clt.num_clt 
from client clt
except  select cd.num_clt  from command cd  group by cd.num_clt  having count(*) > 2;

The PostgreSQL Error is : parser: parse error at or next except 

If I use "(" and ")" for the subquery, the PostgreSQL Error 
is : parser: parse error at or next select.

I try to check "except" with simple query and I don't succeed it.


------- 
QUERY 3
-------

I want to create a view and I want to name the column. 
I don't find the exact syntax for this.

create view payment( NAME_C, TT ) as  select client.name NAME_C, sum(qt*prix) TT  from client, item, command  where
client.num_clt=command.num_cltand item.num_item=command.num_item  group by client.name, client.num_clt
 


An other question : why the num_clt is selected also ?

If I don't name the column, the query is correct and Postgres
chooses the names sum in place of TT. 
(select * from payment returns the correct result) :

create view payment as  select client.name, sum(qt*prix)  from client, item, command  where
client.num_clt=command.num_cltand item.num_item=command.num_item   group by client.name, client.num_clt
 

But when I want to know the client which must pay more than 1000 :

select * from payment 
where sum>1000


The PostgreSQL Error is : rewrite: aggregate column view must be at rigth side in qual


In advance, thank you for your help.

mb