select ip,
       sum(case when score <> (select t2.score from score t2
                               where t2.timestamp = (select max(timestamp) from score
                                                     where ip = t2.ip
                                                       and timestamp < t1.timestamp)
                                 and t1.ip = t2.ip) then 1 else 0 end)
from score t1
group by ip;

select ip, sum(case when score <> s.score) ..
from (select  t2.score ..) s