can method alias another mysql?

i have multi-table query, identical (simplified version)

select columns, count(table2.rev_id) rev_count, sum(table2.rev_rating) sum_rev_rating 
from table1
left join table2
on table1.dom_id = table2.rev_domain_from 
where dom_lastreview != 0 rev_status = 1 
group dom_url 
order sum_rev_rating/rev_count desc

the problem order by clause. causes mysql blunder show, follows:

reference 'sum_ rev_ rating' upheld (reference organisation function)