题解 | #商品交易(网易校招笔试真题)#

```WITH t AS (
	SELECT
		a.*,
		( b.tag_price * sales_num ) AS ori_price 
	FROM
		sales_tb a
		LEFT JOIN product_tb b ON a.item_id = b.item_id 
	WHERE
		substring( sales_date, 6, 2 ) 
	) SELECT
	ROUND( sum( t.sales_price ) * 100 / sum( t.ori_price ), 2 ) AS "discount_rate(%)" 
FROM
	t 
GROUP BY
	DATE_FORMAT( sales_date, "%Y-%m" )