mySql Distinct - group by issue - mysql

Question

I'm trying to find out the most purchased products but to only count distinct users ids. Basically my client wants to stop duplicate purchases from the same user, so that they can't affect the chart/best sellers.
I need to count all order_items for that product, using only Distinct users ids. Currently the results are counting all order_items so the Distinct isn't working.
Any help and I would be grateful.
Thanks in advance
SELECT *
FROM
( SELECT DISTINCT
order_item_meta_3.meta_value as distinct_user_order_items_id,
order_item_meta_2.meta_value as product_id,
SUM( order_item_meta.meta_value ) as item_quantity
FROM
wp_woocommerce_order_items as order_items
LEFT JOIN wp_woocommerce_order_itemmeta as order_item_meta
ON order_items.order_item_id = order_item_meta.order_item_id
LEFT JOIN wp_woocommerce_order_itemmeta as order_item_meta_2
ON order_items.order_item_id = order_item_meta_2.order_item_id
LEFT JOIN wp_woocommerce_order_itemmeta as order_item_meta_3
ON order_items.order_item_id = order_item_meta_3.order_item_id
LEFT JOIN wp_posts AS posts
ON order_items.order_id = posts.ID
LEFT JOIN wp_term_relationships AS rel
ON posts.ID = rel.object_ID
LEFT JOIN wp_term_taxonomy AS tax
USING( term_taxonomy_id )
LEFT JOIN wp_terms AS term
USING( term_id )
WHERE
posts.post_type = 'shop_order'
AND posts.post_status = 'publish'
AND tax.taxonomy = 'shop_order_status'
AND term.slug IN ('completed','processing','on-hold')
AND order_items.order_item_type = 'line_item'
AND order_item_meta.meta_key = '_qty'
AND order_item_meta_2.meta_key = '_product_id'
AND order_item_meta_3.meta_key = '_user_id'
GROUP BY
order_item_meta_2.meta_value
ORDER BY
item_quantity DESC ) as order_table,
wp_posts
LEFT JOIN wp_postmeta as mk1
ON wp_posts.ID = mk1.post_id
LEFT JOIN wp_postmeta as mk2
ON wp_posts.ID = mk2.post_id
WHERE
order_table.product_id = wp_posts.ID
AND wp_posts.ID = mk1.post_id
AND mk1.meta_key = 'is_album'
AND mk1.meta_value = 0
AND mk2.meta_key = '_price'
AND mk2.meta_value = 0

Answer 1

Wouldn't you need to do the sum outside the group by. you have to materialize the distinct set first then aggregate it...
so change first few lines to...
SELECT distinct_user_order_items_Id, product_Id, sum(item_Quantity) as item_Quantity
FROM (
SELECT
order_item_meta_3.meta_value as distinct_user_order_items_id,
order_item_meta_2.meta_value as product_id,
order_item_meta.meta_value as item_quantity