生活中有许多常见的多对一和一对多的场景：老师与学生
student： id,name,tid
teacher： id,name
多对一：查询出所有的学生信息以及学生的老师
SQL语句：
SELECT s.id,s.`name`,t.`name`
from student s,teacher t
where s.tid=t.id;
Bean:
public class Student {
private int id;
private String name;
private Teacher teacher; //注意此处不是tid
public Student() {
super();
// TODO Auto-generated constructor stub
}
public Student(int id, String name, Teacher teacher) {
super();
this.id = id;
this.name = name;
this.teacher = teacher;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
public String toString() {
return "Student [id=" + id + ", name=" + name + ", teacher=" + teacher + "]";
}
}
思考：我们查询结果为Student，要将列映射到属性Teacher，解决列与属性不一致问题
<select id="getStudents" resultMap="studentTeacher">
select s.id sid, s.name sname, t.name tname
from student s,teacher t
where s.tid = t.id;
</select>
<resultMap type="Student" id="studentTeacher">
<result column="sid" property="id"/>
<result column="sname" property="name"/>
<association property="teacher" javaType="Teacher">
<result column="tname" property="name"/>
</association>
</resultMap>
一对多：根据老师的id查询老师信息以及他的所有学生
select t.id tid,t.name tname,s.id sid,s.name sname,s.tid stid
from student s,teacher t
where s.tid=t.id and t.id=1;
Bean:
public class Teacher {
private int id;
private String name;
private List<Student> students;
public Teacher() {
super();
// TODO Auto-generated constructor stub
}
public Teacher(int id, String name, List<Student> students) {
super();
this.id = id;
this.name = name;
this.students = students;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Student> getStudents() {
return students;
}
public void setStudents(List<Student> students) {
this.students = students;
}
@Override
public String toString() {
return "Teacher [id=" + id + ", name=" + name + ", students=" + students + "]";
}
}
<select id="getTeacher" resultMap="teacherStudent">
select t.id tid,t.name tname,s.id sid,s.name sname,s.tid stid
from student s,teacher t
where s.tid=t.id and t.id=#{id};
</select>
<resultMap type="Teacher" id="teacherStudent">
<result column="tid" property="id"/>
<result column="tname" property="name"/>
<collection property="students" ofType="Student">
<result column="sid" property="id"/>
<result column="sname" property="name"/>
<result column="stid" property="tid"/>
</collection>
</resultMap>
总结：
1.遇到查询结果Bean中有对象类型,集合时，使用结果集映射ResultMap
2.ResultMap灵魂：SQL查询列与Bean的属性一一映射，只要懂了这个关系，在复杂的对象，集合都能分解成一个个简单类型
3.关联：association 【多对一】
集合：collection 【一对多】
4.JavaType和ofType都是用来指定对象类型的
JavaType是用来指定pojo中属性的类型
ofType指定的是 映射到list集合属性中pojo的类型
。