select sm.* 
from   system_mail sm 
left join (SELECT u.mail_code 
           from   user_mail u 
           where u.to_user = 1 ) um 
on sm.mail_id = um.mail_code 
where um.id is null.

这个sql在mysql中运行得很好，但是在springdatajpa中运行得不好，我如何在springdatajpa中使用它呢？