Help with ordering a query

Question

Hi,
I have written the following query:
select customer.cust_id as "Customer ID"
     ,customer.cust_area_code as "Customer Area Code"
     ,customer.cust_number as "Customer Number"
     ,sum(calls.elapsed_minutes) as "Total Call Length (Mins)"
     ,sum(calls.elapsed_minutes*calls.rate) as "Total Cost Of Calls £"
from
     (select cust_id
          ,floor((end_date_time-start_date_time)*1440) as elapsed_minutes
          ,chrg_per_minute as rate
     from phonecall
     ) calls
     ,customer
where customer.cust_id = calls.cust_id
group by customer.cust_id, customer.cust_area_code, customer.cust_number, calls.cust_id
order by customer.cust_id asc
However on the last line instead of ordering it by customer.cust_id i would like to order it by "total cost of calls" however when ever i try and order it by this i get an error.
Do i need to assign it to another var before i use it in an order clause?
Thanks

Answer 1

I wrote about [url http://forums.oracle.com/forums/click.jspa?searchID=5648765&messageID=1739843]the order of evaluating on select statement recently.
On the logical, select statement is evaluated in th following order.
[1-1] from
[1-2] on (joining)
[2-1] where (joining)
[2-2] start with, connect by
[2-3] where (excluding joining)
[3] group by
[4] having
[5] select
[6] union, union all, minus, intersect
[7] order by
The alias can be referred predefined in this order, but cannot be referred same level or post-defined. In subquery, this principle is affected as same also.
The table alias and/or the column alias on [1-1] 'from' can be referred all level excluding [1-1], but cannot be referred in inline view (subquery) on [1-1] 'from'.
select xxx from tab1 a1,(select xxx2 from tab2 a2 where a2.xxx=a1.xxx) -- No Good (This is same level)
select a1,a2 from (select xxx a1, xxx2 a2 from tab1) -- Good
select * from (select xxx a1, xxx2 a2 from tab1) -- Good, * explode to a1,a2
The column alias on [5] 'select' only can be referred on [7] 'order by'.
select xxx a1, xxx2 a2,count(*) from tab1 group by a2 -- No Good (post defined)
select xxx a1, xxx2 a2,count(*) from tab1 order by a2 -- Good
The naming rule of alias must accord with [url http://download-west.oracle.com/docs/cd/B19306_01/server.102/b14200/sql_elements008.htm#i27561]Schema Object Names and Qualifiers.
So, the alias enclosed by double quotation mark becomes case sensitive.
In additional, in order-by-clause, we can specified column number on select-clause.
select empno,ename,sal from scott.emp order by 3; -- this means order by sal