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 k.bedrijfsnaam 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!