consulta sql para diferença entre a linha atual e a linha anterior com base na data e hora

select t.*,
       coalesce(t.cumulativeValue - tprev.cumulativeValue, t.cumulativeValue) as diff
from t outer apply
     (select top 1 tprev.*
      from t tprev
      where tprev.siteId = t.siteId and tprev.readtime < t.readtime
      order by tprev.readtime desc
     ) tprev;