用开窗函数代替子查询优化以下mysql代码：select item_category,sum(amount) as 销售额, sum(sum(amount)) over (order by sum(amount) desc) as 累计销售额, sum(sum(amount)) over (order by sum(amount) desc)/ (select sum(amount) from userbehavior_new where behavior_type='buy') as 累计销售额占比 from userbehavior_new group by item_category order by 销售额 desc;

使用开窗函数代替子查询优化后的MySQL代码如下：

SELECT item_category,
       SUM(amount) AS 销售额,
       SUM(SUM(amount)) OVER (ORDER BY SUM(amount) DESC) AS 累计销售额,
       SUM(SUM(amount)) OVER (ORDER BY SUM(amount) DESC) / buy_sum AS 累计销售额占比
FROM userbehavior_new
CROSS JOIN (
    SELECT SUM(amount) AS buy_sum
    FROM userbehavior_new
    WHERE behavior_type = 'buy'
) AS T
GROUP BY item_category
ORDER BY 销售额 DESC;

优化点：
1. 使用开窗函数SUM(SUM(amount)) OVER (ORDER BY SUM(amount) DESC)将累计销售额计算为一个窗口函数，以避免子查询。
2. 使用CROSS JOIN将子查询中的总购买金额与每个item_category的销售额进行关联。
3. 保持原有的GROUP BY和ORDER BY子句不变。