select * from c left join b on b.hetonghao=c.hetonghao 
inner join a on a.hetonghao=b.hetonghao
where (a.danjia*b.shuliang) <>b.huikuanjine

------解决方案--------------------
SQL code

select c.hetongNo
from c join (select hetongNo,sum(price*num) amount from a group by hetongNo) b on c.hetongNo = b.hetongNo
       join (select hetongNo,sum(reAmount) amount from b group by hetongNo) d on c.hetongNo = d.hetongNo
where b.amount <> c.amount
group by c.hetongNo

------解决方案--------------------
SQL code

select distinct c.合同号 from c left join a on c.合同号=a.合同号 left join b on c.合同号=b.合同号
group by c.合同号
having sum(a.单价*数量）<>sum(b.回款金额）

------解决方案--------------------
这个错了，呵呵。
探讨

SQL code


select distinct c.合同号 from c left join a on c.合同号=a.合同号 left join b on c.合同号=b.合同号
group by c.合同号
having sum(a.单价*数量）<>sum(b.回款金额）


------解决方案--------------------
select distinct c.合同号
from a,b,c
where a.合同号=b.合同号
and b.合同号=c. 合同号
and b.回款金额 NOT (a.单价 * a.数量)

------解决方案--------------------

select distinct a.Contractno from A
where a.Contractno not in (
select d.Contractno(
       (select b.Contractno,sum(b.price*b.num) as totalb from B b group by b.Contractno) d  
  left join (select c.Contractno,sum(c.backprice) as totalc from C c group by  
c.Contractno) e  on e.Contractno = d.Contractno
where d.totalb=e.totalc
)