MySQL 删除重复数据实例

如何删除重复数据

 

业务场景：删除评论表中对同一订单同一商品的重复评论，只保留最早的一条。

1. 查看是否存在对于同一订单同一商品的重复评论。

 

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

 

2. 备份product_comment表。

CREATE TABLE bak_product_comment_18051801 LIKE product_comment;

INSERT INTO bak_product_comment_18051801 SELECT * FROM product_comment;

 

3. 删除同一订单的重复评论。

DELETE a
FROM product_comment a
JOIN(
    SELECT order_id,product_id,MIN(comment_id) AS comment_id
    FROM product_comment
    GROUP BY order_id,product_id
    HAVING COUNT(*)>=2
) b ON a.order_id=b.order_id AND a.product_id=b.product_id
AND a.comment_id>b.comment_id