sql server 2008 - Finding Max Revision -

i have table similar below:

req     po      line    cost    rev 101630  p101630 1       26.48   0 101630  p101630 2       26.82   0 101630  p101630 3       17.83   0 101630  p101630 1       26.48   1 101630  p101630 2       110.00  1 101630  p101630 3       17.83   1 101630  p101630 1       52.96   2 101630  p101630 2       110.00  2 101630  p101630 3       35.66   2 

how find result want this:

req     po      line    cost    rev 101630  p101630 1       52.96   2 101630  p101630 2       110.00  2 101630  p101630 3       35.66   2 

i tried using didn't return correct results

select  req, po, line, cost, max(rev) table po= 'p101630' group req, po, line, cost 

i use (if want last revision each single req, po, line):

select  req, po, line, cost (     select req, po, line, cost, row_number() on (partition req, po, line order rev desc) rn     table ) rn = 1