在HQL中使用count的问题

我的user表中有个class(班级)列，我想以班级的人数分组查询 select count(*) as a from user group by class order by a desc 我用hibernate做的HQL select c.count(c),c.class from User c group by c.class session.createQuery(sql).list()却报错了

javax.servlet.ServletException: No data type for node: org.hibernate.hql.ast.MethodNode +-[METHOD_CALL] MethodNode: '(' | \-[METHOD_NAME] SqlNode: 'exprList' | \-[ALIAS_REF] IdentNode: 'user0_.userid' {alias=c, className=com.fuqi.dao.pojo.user, tableAlias=user0_}

请问这是怎么回事啊？急待。。。