How To Do Percent/Total in SQL?

Question

I have an typical CUSTOMER/ORDERS set of tables and I want to display the total percentage of sales a particular customer is responsible for. I can get the

Answer 1

select abc.item_name, sum(amount) as total
  from (select a.item_id, d.applicablefrom, a.item_name, a.final_item_status, d.rate, c.item_name as sub_item_name,
               b.sub_item_qty as itemqty, (b.sub_item_qty * d.rate) as amount 
          from tblitem_master a,
               tblitem_master c,
               tblitem_bom_master b,
               (select rate, applicablefrom, itemid
                  from tblperiodrates
                 where applicablefrom = (select max(applicablefrom) 
                                           from tblperiodrates
                                          where applicablefrom<='2005-5-18')) as d
         where a.item_id = b.item_id
           And b.sub_item_id = c.item_id
           and b.sub_item_id = d.itemid
           and a.final_item_status='f') as abc
 group by abc.item_name