Question

I’m building a MySQL query but I can’t seem to get it right.

I have four tables:
– customers
– orders
– sales_rates
– purchase_rates

There is a 1:n relation ‘customernr’ between customers and orders.
There is a 1:n relation ‘ordernr’ between orders and sales_rates.
There is a 1:n relation ‘ordernr’ between orders and purchase_rates.

What I would like to do is produce an output of all customers with their total purchase and sales amounts.

So far I have the following query.

SELECT c.customernr, c.customer_name, SUM(sr.sales_price) AS sales_price, SUM(pr.purchase_price) AS purchase_price
FROM orders o, customers c, sales_rates sr, purchase_rates pr
WHERE o.customernr = c.customernr
AND o.ordernr = sr.ordernr
AND o.ordernr = pr.ordernr
GROUP BY c.customer_name

The result of the sales_price and purchase_price is far too high. I seem to be getting double counts. What am I doing wrong? Is it possible to perform this in a single query?

Thank for your response!

Answer 1

It seems that the problem is that when you join the orders table to the tables with sales rates and purchase rates, you are getting the cartesian product of these two latter tables. I.e each row in these two tables are repeated once for each correponding row in the other table. The following query should solve this problem by summing the rates for each order before joining the sales rates and purchase rates to the other tables:

SELECT c.customernr, c.customer_name,
       SUM(sr.sales_price) AS sales_price,
       SUM(pr.purchase_price) AS purchase_price
FROM customers c
INNER JOIN orders o
ON o.customernr = c.customernr
LEFT JOIN (SELECT ordernr, SUM sales_price) AS sales_price
           FROM sales_rates
           GROUP BY ordernr) sr
ON sr.ordernr = o.ordernr
LEFT JOIN (SELECT ordernr, SUM(purchase_price) AS purchase_price
           FROM purchase_rates
           GROUP BY ordernr) pr
ON pr.ordernr = o.ordernr
GROUP BY c.customernr, c.customer_name;