一对一：<association></association>
一对多：<collection></collection>
ClassMapper.xml:
<resultMap type="com.taobaoo.school.domain.edu.XjClass" >
select
cls.id AS class_id,
cls.class_name AS class_name,
school.id AS school_id,
school.school_name AS school_name,
grade.id AS grade_id,
grade.grade_name AS grade_name,
teacher.id AS class_teacher_id,
teacher.name AS class_teacher_name
from class cls
join grade on cls.grade_id=grade.id
join school on cls.school_id=school.id
join teacher on cls.teacher_id=teacher.id
</select>
Class.java:
private Long id;
private Grade grade; //1对1
private Teacher teacher; //1对1
Private List<Teacher> teaList; //1对多
//get/set省略...