mysql的delete語句使用exists删除資料走不通

mysql中，打算使用exists查找相關記錄，然後删除，結果搞不定。

我們有個表folder，裡面有個字段code存在重複值，本來編碼應該是唯一的。是以想将重複記錄删除。

這樣

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

不行！

這樣

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));      

還是不行！

查找​​網上的例子​​：

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;