hql语句，Path expected for join问题

[santhe]

hql语句是这样的 ：select max(s.sequenceNum) from Student as s left join GroupUserRelation as gur where gur.userGroup = ?1 and gur.role = ?2

student 是 user的子类， user是 GroupUserRelation 的属性

但是执行语句的时候报 Path expected for join

高手请帮忙看看是什么问题