按类别（总价值和销售数量）报告2015年产品的销售情况。还要确定一个给定类别的销售额的百分之几代表该类别中每个产品的销售额。
我目前的问题是：

WITH sales AS
            (SELECT t1.category_name
            ,       t2.product_name
            ,      (t3.unit_price*t3.quantity) Total_sales
            ,      EXTRACT (YEAR FROM order_date) Year
            FROM categories t1
                    INNER JOIN
                 products t2 
                    ON t2.category_id=t1.category_id
                    INNER JOIN
                 order_details t3
                    ON t3.product_id=t2.product_id
                    INNER JOIN
                 orders t4
                    ON t4.order_id=t3.order_id
            WHERE EXTRACT (YEAR FROM order_date) = '2015'
            GROUP BY  t1.category_name
            ,         t2.product_name
            ,         (t3.unit_price*t3.quantity)
            ,         EXTRACT (YEAR FROM order_date)
            ORDER BY 1
                    )
SELECT s.category_name
,      s.product_name
,      SUM (Total_sales)
FROM sales s
GROUP BY s.category_name
,        s.product_name
ORDER BY 1

如何计算%？谢谢您