"INNER JOIN" and "LEFT JOIN" with "GROUP BY" - mysql

Question

My tables looks like this
sales
----------------------------------------------------------
id ordernumber quantity category_id price
1 402-9182243-8008368 1 3 22.95
2 406-3666671-8627555 2 3 6.95
3 303-1935495-5532309 1 1 7.95
4 171-5799800-1198702 1 2 159.95
5 403-2398078-4901169 2 2 18.95
category
--------------
id name
1 bikes
2 shoes
3 planes
returns
--------------
id ordernumber quantity costs
1 402-9182243-8008368 1 22.95
2 402-9182243-8008368 5.95 // return shipping fee
And here is my query
SELECT c.name,
SUM(v.quantity) AS sold, # wrong
SUM(s.quantity * s.price) AS turnover, # wrong
SUM(r.costs) AS returncosts,
FROM sales AS s
INNER JOIN categories AS c ON c.id = s.category_id
LEFT JOIN returns AS r ON r.ordernumber = s.ordernumber
GROUP BY c.name
I have some inner joins with aggregate functions.
But I also need "return" with a "Left Join" (I think).
And with Left Join, my aggregate functions dont work anymore.
Left Join adds additional rows. Additional data, for sum().
I need a single query, so every column is sortable later.
I would be happy about any help. Best Regards

Answer 1

It's a semi cartesian product because ordernumber is not unique in returns table.
We can see what's happening if we remove the aggregate functions and return the detail rows.
One possible approach is to pre-aggregate returns in an inline view, so that unique values of ordernumber are returned.
Assuming ordernumber is unique in sales table, then something like this:
SELECT c.name
, SUM(s.quantity) AS sold
, SUM(s.quantity * s.price) AS turnover
, SUM(r.returncosts) AS returncosts
FROM sales s
JOIN categories c
ON c.id = s.category_id
LEFT
JOIN ( SELECT t.ordernumber
, SUM(t.costs) AS returncosts
FROM returns t
GROUP
BY t.ordernumber
) r
ON r.ordernumber = s.ordernumber
GROUP
BY c.name

Answer 2

You can sum the quantity separately from the LEFT JOIN in a sub query as follows:
SELECT t1.name, t1.sold, t1.turnover, SUM(r.costs) AS returncosts
FROM(
SELECT c.name,
SUM(s.quantity) AS sold,
SUM(s.quantity * s.price) AS turnover
FROM sales AS s
INNER JOIN categories AS c ON c.id = s.category_id
GROUP BY name
) t1
LEFT JOIN returns AS r ON r.ordernumber = s.ordernumber
GROUP BY t1.name, t1.sold, t1.turnover