MySQL double group - mysql

Question

I am making queries to extract data from database which holds customer order. There's one table which holds customer id's and the customer's name. Another table which has the order id, customer id of who placed the order, a quantity of the item bought, and an item id. The last table holds the item id's and item names. I am trying to sort these to show an individual's most popular purchase, but am having issues properly grouping and ordering to produce the correct result, below is an example of what is intended.
customers
1 | John
---+-----
2 | Jane
orders
1 | 2 | 4 | 1
---+---+---+---
2 | 2 | 5 | 2
---+---+---+---
3 | 2 | 2 | 1
---+---+---+---
4 | 1 | 1 | 2
items
1 | Chair
---+-------
2 | Sofa
After properly sorting and grouping, the output table should like:
John | Sofa
------+------
Jane | Chair
Currently I can connect the item names to the purchaser and return a random item bought, but not the most popular by quantity. I have tried entering multiple fields into group by and managed to properly group the items by name and sort by quantity, but in doing so the customer id's became ungrouped. Been trying to solve this for days so any help would be appreciated. Please note that this is a very simplified version of the actual problem where many more tables are involved, including multiple items table which are being joined together to one.

Answer 1

You should use group by on joined tables
select
b.name
, c.name
, sum(quantity) as tot
from orders as a
inner join Customers as b on a.customer_id = b.id
inner join Items as c on a.item_id = c.id
group by b.name, c.name
order by tot

Answer 2

Selecting the sum of the quantities per customer-item group is easy, but selecting the top seller is a bit harder.
The first step is the query to get all the groups with the sums of the quantities for each customer-item:
SELECT
customer_name,item_name,SUM(quantity)
FROM
orders o
JOIN customers c ON o.customer_id=c.id
JOIN items i ON o.item_id=i.id
GROUP BY customer_name,item_name;
Then to only select the groups with the maximum quantity sums we use some trickery:
SELECT
customer_name,item_name,SUM(quantity),
(SELECT SUM(quantity) AS qmax
FROM
orders o2
JOIN customers c2 ON o2.customer_id=c2.id
JOIN items i2 ON o2.item_id=i2.id
WHERE c2.id=c.id
GROUP BY c2.customer_name,i2.item_name
ORDER BY qmax DESC LIMIT 1) AS qmax
FROM
orders o
JOIN customers c ON o.customer_id=c.id
JOIN items i ON o.item_id=i.id
GROUP BY customer_name,item_name
HAVING SUM(quantity)=qmax;
Edit:
Here's a link to a fiddle: SQLFiddle