MySql中删除重复垃圾记录

1. 将需要的数据去重（利用group by）查询出

2. 删除原表中不再里面的记录

a. 重复记录由多个字段判断
 查询出重复记录：
        select id from user group by userName, password having count(*) >=1);
  删除
      delete from user where id not in (select id from (select id from user group by userName, password having count(*) >=1) u);
b. 重复记录由一条字段组成

  查询出重复记录：
        select id from user group by id having count(*) > =1;

删除
              delete from user where id not in (select id from (select id from user group by id having count(*) > =1) u);

引用临时表是为了防止mySql中的You can't specify target table 'user' for update in FROM clause错误