我的困惑围绕以下几点：
SELECT t3.id, t3.name SUM(t1.quantity_available) as 'quantity' FROM product_merchant as t1
INNER join product_category as t2
(inner join category as t3
on t2.category_id=t3.id on t1.product_id = t2.product_id )
where t1.merchant_id= 3 GROUP BY t3.id