Thanks guys, Draoued code works well. I also got a variation using 'CASE WHEN...THEN....ELSE...END' as suggested by drlex. For anyone wanting to use Draoued's code, the compiler requests a couple more brackets. I've shown the final code below.
(select /*+ FIRST ROWS */ "Request"."REQUESTID" "Request_ID"
from "ECASH"."REQUEST" "Request" where "Request"."VOID"= decode((#prompt('choice')#), '0' , 0 , 1 ))