mysql的delete语句使用exists删除数据走不通

mysql中，打算使用exists查找相关记录，然后删除，结果搞不定。

我们有个表folder，里面有个字段code存在重复值，本来编码应该是唯一的。因此想将重复记录删除。

这样

delete from folder fd1 where exists(
select 1 from folder fd2 where id<>fd1.id and code=fd1.code);      

不行！

这样

delete from folder where id in(
select id from folder fd1 where exists(
select 1 from folder fd2 where id<fd1.id and code=fd1.code));      

还是不行！

查找​​网上的例子​​：

DELETE tb
    FROM tb LEFT JOIN
         tb tb2
         ON tb2.`merchantId` = 'A32WNPGI8GE4WW' AND
            tb2.`marketplaceId` IN ('A1AM78C64UM0Y8', 'A2EUQ1WTGCTBG2', 'ATVPDKIKX0DER')
    WHERE tb2.merchantID IS NULL;      

create temporary table tmp_folder_id as select id from folder fd1 where exists(select 1 from folder fd2 where id<fd1.id and code=fd1.code);

delete from folder where id in(select id from tmp_folder_id);

drop table tmp_folder_id;