rownum理解

select *
  from (select rownum bb,aa.*
          from (select bb.*, cf.dtdiscountdate, cf.mdiscountrate
                  from Loan_DiscountContractBill bb, loan_contractform cf
                 where 1 = 1
                   and bb.ncontractid = cf.id
                   and to_char(bb.dtEnd,'yyyy-mm-dd') > '2010-03-31'
                      -- (select to_char(sysdate+15,'yyyy-mm-dd') from dual)
                   and bb.nContractId > 0 
                   and bb.nstoragestatusid = 1
                 order by bb.nContractID, bb.nSerialNo) aa)