select query in oracle -

i have database design this...

   dated       ref       weight  no.  address  21-03-2013  abcd/edfg     1234   a45   a1  20-03-2013  abcd/edfg     789    a56   a2 25-03-2013  abcd/edfg     6981   a99   a5  23-03-2013  gajhs/asdh     72    a82   gv 

what want here in query result this....
search on basis of no.
then, has see if ref of row exists more times , if exists has add weight of such rows keeping in mind dated of such records should less dated of selected no..

example -

no. = a56 3 rows exist same ref(abcd/edfg) dated of a56 lower among results should        ref ------ weight --------  no. -------- address abcd/edfg ------ 789 ----------- a56 -------- a2 

but in case of no. = a99 results should -

ref ----------- weight --------  no. -------- address abcd/edfg --- (789+6981+1234) ----------- a99 -------- a5   dated of a99 greater other 2 records. 

please me in query.

select     ref,     sum(weight) on (partition ref order dated) weight,    no,    address    ...