如何使用jpql检索许多列？

Question

我在从这个类中检索一些信息时遇到问题。

@Entity
public class Absence {
@Id @GeneratedValue(strategy= GenerationType.IDENTITY)
private int absenceId;
private int crn;
private int studentId;
@Temporal(TemporalType.DATE)
private Date absenceDate;

}

我想检索以下信息：studentId，学生姓名，缺席人数。查询不起作用的问题。 我正在使用的查询是

public List<StudentAbsenceSummary> getSetionAbsences(int crn) {
Query q=em.createQuery("select b.studentId,count(b.studentId)   from Absence b   where            b.crn =:crn group by (b.studentId)")
            .setParameter("crn", crn);
    List<StudentAbsenceSummary> students =q.getResultList();
    return students;
}

检索到的列表的类

public class StudentAbsenceSummary {
private int studentId;
private String name;
private int absenceCount;