mySQL - NL

SELECT max(oim2.meta_value/oim3.meta_value) FROM `wp_woocommerce_order_itemmeta` as oim
INNER join wp_woocommerce_order_items as oi

on oim.order_item_id = oi.order_item_id

inner join wp_woocommerce_order_itemmeta as oim2

on oim.order_item_id = oim2.order_item_id

inner join wp_woocommerce_order_itemmeta as oim3

on oim.order_item_id = oim3.order_item_id

inner join wp_postmeta as pm
ON pm.post_id = oi.order_id

where pm.meta_key = '_customer_user' and pm.meta_value = 14

and oim.meta_key = '_product_id'

and oim.meta_value = 215

and oim2.meta_key = '_line_subtotal'

and oim3.meta_key = '_qty'