相关疑难解决方法(0)

我正在使用JPA namedQuery从DB中选择数据.

 @NamedQuery(name = "Concept.findByRefTableNull", query = "SELECT c FROM Concept c WHERE c.conceptName = :conceptName and c.refTable = :refTable"),

///
 List<Concept> attributeList 
                = em.createNamedQuery("Concept.findByRefTableNull")
                .setParameter("conceptName", "student").
                setParameter("refTable", null).
                getResultList();

        System.out.println(attributeList.size()); //return 0

列表大小为0,但我确信它应该有记录.原因是refTable.如何在JPA中查询哪个值为null的列？

谢谢.