数据库 sql需求

1 需求：某人借钱（A表），还钱（B表） 要查询这个人还欠多少钱？，1次借钱可能对应多次还钱 （1-- 0.....n）
   方法1
  

select a.app_id, sum(a.借钱), sum(b.还钱)
  from A
  left join on B on a.id_A = b.id_A
 group by a.id_App

以上是错误的

 select a1, a2, sum(借钱), max(常量), sum(还钱)
  from (select A.a1 as a1, A.a2 as a2, 借钱 as 借钱, '', 0 as 还钱
          from A
        union all
        select A.a1 as a1, A.a2 as a2, 0 as 借钱, b.常量, b.还钱 as 还钱
          from A, B
         where A.id_A = B.id_A) C