如何防止休眠中的空列表错误？答案

【问题标题】：How to prevent empty List Error in Hibernate?如何防止休眠中的空列表错误？
【发布时间】：2013-07-09 19:49:48
【问题描述】：

我在休眠中有以下 HQL 查询：

select x 
from Item as x
where x not in (select o.item from Opinion as o where o.user =:user)

除非子查询 (select o.item from Opinion as o where o.user =:user) 返回一个空列表，否则此查询工作正常。在这种情况下，我得到了一个错误。

当子查询 (select o.item from Opinion as o where o.user =:user) 为空时，有没有办法防止 Hibernate 引发错误？

我如何必须重写查询以使其即使对于空 (select o.item from Opinion as o where o.user =:user) 也能正常工作？

【问题讨论】：

【解决方案1】：

我看到的一个简单的方法是

select x 
from Item as x
where (select count(distinct o.item) from Opinion as o where o.user =:user) = 0

或（*未经测试）

select x 
from Item as x
where not exists (from Opinion as o where o.item = x and o.user =:user)

【讨论】：