with a as(
  select 2 id, 900  value from dual union all
  select 3 id, 1000 value from dual union all
  select 5 id, 860  value from dual union all
  select 4 id, 1780 value from dual union all
  select 8 id, 808  value from dual),
b as(
  select 2 id, 900  value from dual union all
  select 3 id, 1000 value from dual union all
  select 7 id, 860  value from dual union all
  select 1 id, 1780 value from dual union all
  select 8 id, 808  value from dual union all
  select 10 id, 1000 value from dual)
  
  SELECT nvl(b.id, a.id) id,
         CASE
           WHEN a.id IS NOT NULL AND b.id IS NOT NULL THEN
            b.value - a.value
           WHEN a.id IS NOT NULL THEN
            -a.value
           ELSE
            b.value
         END VALUE
    FROM a
    FULL OUTER JOIN b
      ON a.id = b.id;