SELECT products.product_id, products.product_description @entries := (IFNULL ((SELECT Sum(purchase_items.purchased_items_quantity) FROM purchase_items WHERE purchase_items.product_id = products.product_id),0)) AS stock_entries, @outs:= (IFNULL ((SELECT Sum(sale_items.sold_items_quantity) FROM sale_items WHERE sale_items.product_id = products.product_id),0)) AS stock_outs, (@entries - @outs) AS final_stock FROM products
Para o Knex?