In this example:

select customer_id,customer_nme,(select sum(sell_amount) from sales where sales.customer_id=customer.customer_id) as sales_total
from customer
where sales_total>0
order by 3,2 desc

Why do I get the error message: Invalid column name 'sales_total'?
Is there a way around this without having to repeat the query in the where command (similar to the order command where you just specify the column number)?