query needed

[VenuGopal Papasani]

Hi,
 i got a table datavalue as follows
              code             period               value

              c1                  2005                1
               c2                 2005                2
                c3                2006                 3
               c4                   2005               2
                c5                2005                   1
         now i need a query where some values should be added and some
values should be subtracted of certain period.for ex here 2005 now i need
(c1+c2-c4-c5)              can i do it in a single query .Can any one give
me the query plsssssssssss


regards,
venu.