MySQL use DISTINCT with GROUP BY on same column - mysql

Question

I currently have a query similar to:
select customers.customerId, items.itemName, sum(orders.quantity) as boughtTotal
from customers join orders on customers.customerId = orders.customerId
join items on items.itemId = orders.itemId
group by customers.customerId, items.itemName
order by boughtTotal desc;
The purpose of the query is to relate customers to items they bought, which may span over many orders, and total the amount of each unique item bought. This being achieved with what I have. I want to take this a step further now and select the most popular item for each customer. Since this is ordered with most bought items at the top, I figured I'd just have to add distinct next to customers.customerId in the select statement to have duplicates removed. However, adding distinct seems to do nothing. I'd appreciate help in knowing why distinct is seemingly doing nothing here, but also how to achieve what I'm trying to do - remove duplicates besides a customer's most popular item.
Tables:
customers
customerId | name
1 | John
2 | Jane
orders
orderId | customerId | quantity | itemId
1 | 1 | 11 | 1
2 | 2 | 13 | 2
3 | 1 | 4 | 2
4 | 2 | 14 | 1
5 | 1 | 1 | 1
items
itemId | itemName
1 | dog
2 | cat
So from this data the current query will return the following:
customerId | itemName | boughtTotal
2 | dog | 14
2 | cat | 13
1 | dog | 12
1 | cat | 4
And what I'd like to have is the following:
customerId | itemName | boughtTotal
2 | dog | 14
1 | dog | 12

Answer 1

Try this;)
select t1.*
from (
select customers.customerId, items.itemName, sum(orders.quantity) as boughtTotal
from customers join orders on customers.customerId = orders.customerId
join items on items.itemId = orders.itemId
group by customers.customerId, items.itemName) t1
inner join (
select max(boughtTotal) as boughtTotal, customerId
from (
select customers.customerId, items.itemName, sum(orders.quantity) as boughtTotal
from customers join orders on customers.customerId = orders.customerId
join items on items.itemId = orders.itemId
group by customers.customerId, items.itemName)t
group by customerId) t2 on t1.customerId = t2.customerId and t1.boughtTotal = t2.boughtTotal
DEMO HERE