sql语句，求高手指教,该如何解决

sql语句，求高手指教
表A id , value
  2 900
  3 1000
  5 860
  4 1780
  8 808


表B id , value
  2 900
  3 1000
  7 860
  1 1780
  8 808
  10 1000

我要让表B减表A 得到对应的id和value这两列
 
当B.id=A.id 时 id=B.id , value=B.value-A.value
当B.id在A里面没有时 id=B.id value=B.value
当A.id在B里面没有时 id=A.id value=-A.value


sql语句怎么写，高手指教

[解决办法]

SQL code

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;