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SELECT order_id, product_id, COUNT(*) 
		FROM product_comment 
	GROUP BY order_id, product_id 
	HAVING COUNT(*)>1;

-- 使用LIKE操作完整複製product_comment表的建表語句
CREATE TABLE bak_product_comment_180110 
	LIKE  product_comment;

-- 插入原表的資料
INSERT INTO bak_product_comment_180110 
	SELECT * FROM product_comment;
 

DELETE a FROM product_comment AS a JOIN (
	SELECT order_id, product_id, MIN(comment_id) 
		FROM product_comment GROUP BY order_id, product_id HAVING COUNT(*)>1
) AS b ON a.order_id=b.order_id AND a.product_id=b.product_id AND a.comment_id>b.comment_id;