sql sum函数得到错误的结果

Question

select 
    c.c_name, 
    sum(s.net_amount) - sum(cp.recived_amount) as amount 
from customer_payments cp
inner join customer c 
    on c.c_id = cp.customer_id 
inner join sale s 
    on s.cId = c.cId 
group by 
    c.c_name 
having 
    sum(cp.net_amount) - sum(recived_amount) !=0

I have 3 tables:

Customer(cId)(cName)
Sale(cId)(net_amount)
Customer_payment(cId)(received_amount)

I want to sum net_amount and received_amount and subtract them both. Through inner join whem I am going to sum both rows it is not giving correct result.

Answer 1

One problem is that you are getting a cartesian product product for each customer. Another is that you are possibly losing rows when there are no matches.

You can fix these problems by using left join and pre-aggregating the values:

select c.c_name, sum(s.net_amount) - sum(cp.received_amount) as amount
from customer c left join
     (select cp.customer_id, sum(cp.received_amount) as receivedamount
      from customer_payments cp
      group by cp.customer_id
     ) cp 
     on c.c_id = cp.customer_id left join
     (select s.cId, sum(s.net_amount) as net_amount
      from sale s
      group by s.cId
     ) s
     on s.cId = c.cId
group by c.c_name
having sum(cp.net_amount)-sum(received_amount) <> 0;

The outer aggregation is not really needed, unless a given customer has multiple id for the same name.