For each product, find the customer that purchases the most

gistfile1.txt

select productCode as p, customerNumber as c from orders join orderdetails
  on orders.orderNumber = orderdetails.orderNumber
  group by productCode, customerNumber
  having (productCode, customerNumber) = (
	select productCode, customerNumber
	from orders join orderdetails ON orders.orderNumber = orderdetails.orderNumber
	where productCode = p
	group by productCode, customerNumber
	order by sum(quantityOrdered) desc
	limit 1
  )

gistfile2.txt

select * from (
  select productCode, sum(quantityOrdered) as totalOrdered from orderdetails
  group by productCode
  ) as t1
 JOIN (
   select productCode as p, customerNumber as c from orders join orderdetails
  on orders.orderNumber = orderdetails.orderNumber
  group by productCode, customerNumber
  having (productCode, customerNumber) = (
	select productCode, customerNumber
	from orders join orderdetails ON orders.orderNumber = orderdetails.orderNumber
	where productCode = p 
	group by productCode, customerNumber
	order by sum(quantityOrdered) desc
	limit 1
  )
   ) as t2
  on t1.productCode = t2.p