项目中比较常用的数据筛选场景

一、如何删除重复的数据

删除重复数据，对于相同数据保留ID 最大的记录
数据库的数据为：

SELECT * from `user` ;

询重复数据的 id 值：

SELECT user_name , count(*),max(id) from user GROUP BY user_name HAVING count(*)>1;

删除语句

DELETE a from `user` a JOIN(
SELECT user_name , count(*),max(id) as id from user GROUP BY user_name HAVING count(*)>1
) b on a.user_name=b.user_name where a.id<b.id