merge into t
using (
  with new as (
    select null as item, null as value, null as moment from dual
    union all select 'item1', 10, sysdate from dual
    union all select 'item2', 20, sysdate from dual
  ), d as (
    select null as flag from dual
    union all select 1 from dual
  )
  select new.item, new.value, new.moment, d.flag
  from new, d
) rs on (t.item = rs.item and t.date_beg <= rs.moment and (t.date_end is null or t.date_end > rs.moment) and rs.flag is not null)
when matched then update set t.date_end = rs.moment
when not matched then insert (item, value, date_beg) values (rs.item, rs.value, moment)