表A：
ID      Qty
10      0
101     100
102     200
20      0 
201     500

表B：
ID      ParentID
101     10
102     10
201     20 

现在需要得出如下结果：
ID      Qty
10      300
101     100
102     200
20      500
201     500

-- 建表
create table ta(id int, qty int);
insert into ta 
select 10,0 union all
select 101,100 union all
select 102,200 union all
select 20,0  union all
select 201,500;

create table tb(ID int, ParentID int);
insert into tb 
select 101,10 union all
select 102,10 union all
select 201,20;
-- 查询
select id, 
qty=case when (select COUNT(*) from tb where tb.ParentID=ta.id)>0 
    then isnull((
        select sum(c.qty) from ta c where c.id in(select id from tb where parentid=ta.id) 
        ),0)
    else qty end
from ta

-- 结果
10    300
101    100
102    200
20    500
201    500

------解决方案--------------------


select a.id,Sum(Case when b.ParentID Is Null then a.qty else c.qty End) from ta a
left outer join tb b on a.id=b.ParentID
left outer join ta c on b.id=c.id
Group by a.id