select GREATER(t1.startperiod, t2.startperiod) as x_startperiod
     , LESSER(t1.endperiod, t2.endperiod) as x_endperiod
     , case
         when t1.endperiod > t2.endperiod
           then t2.value2
         else null 
       end as value2
     , t1.value1
  from t1
  join t2
    on t1.endperiod >= t2.startperiod
   and t2.endperiod >= t1.startperiod
 order by x_startperiod

select GREATER(t1.startperiod, t2.startperiod) as x_startperiod
     , LESSER(t1.endperiod, t2.endperiod) as x_endperiod
     , t3.value2, t1.value1
  from t1
  join 
     (-- Свёртка пересекающихся периодов (T2):
       select min(startperiod) as startperiod, max(endperiod) as endperiod, min(value2) as value2
         from 
            (
              select startperiod, endperiod, value2
                   , sum(sog) over(order by startperiod,endperiod) as grp_id
               from 
                  (
                    select startperiod, endperiod, value2,
                           case 
                             when max(endperiod) 
                                 over(order by startperiod, endperiod 
                                       rows between unbounded preceding
                                                and 1 preceding) >= startperiod
                               then 0 
                             else 1 
                           end as sog -- sog = start_of_group :)
                      from t2
                  )
            )
          group by grp_id
       )t2
    on t1.endperiod >= t2.startperiod
   and t2.endperiod >= t1.startperiod
  left join t2 t3
    on LESSER(t1.endperiod, t2.endperiod) = t3.endperiod
 order by x_startperiod