select distinct sum(DATEDIFF(hour, min(FromDateTime), dateadd(hh,1,max(ToDateTime)))) over() as x_value
  from (
         select  FromDateTime, ToDateTime, sum(sog) over(order by FromDateTime,ToDateTime) as grp_id
           from (
                  select FromDateTime, ToDateTime,
                         case 
                           when max(ToDateTime) 
                               over(order by FromDateTime, ToDateTime 
                                        rows between unbounded preceding
                                                         and 1 preceding
                                   ) >= FromDateTime
                           then 0 
                           else 1 
                         end as sog 
                    from uchet_vremeni t
                ) v0
       ) v1
 group by grp_id

sum(sog) over(order by FromDateTime,ToDateTime) as grp_id

sum(sog) over(order by FromDateTime,ToDateTime) as grp_id