数据库实体 多对一实现 和一对多

多对一
public class Student {
    private int id;
    private String name;
    private Teacher teacher;// 表示 student 对 teacher是多对一。

public class Teacher {
    private int id;
    private String name;

表结构如下：

   <!--多对一查询 -->
    <select id="getStudents2" resultMap="studentTeacher">
            select s.id sId, s.name sName ,t.id tId,t.name tName  from student s, teacher t
            where s.tid = t.id
    </select>
    <!-- 多对一查询 方法一：使用级联属性的方式
    因为 tId是 Teacher类的 id属性，而 Student类有Teacher类的属性叫 teacher
    所以可以属性级联：teacher.id
     -->
    <resultMap id="studentTeacher" type="Student">
        <result property="id" column="sId"></result>
        <result property="name" column="sName"></result>
        <result property="teacher.id" column="tId"></result>
        <result property="teacher.name" column="tName"></result>
    </resultMap>

　　

方法二：
<resultMap id="studentTeacher2" type="Student">
<result property="id" column="sId"></result>
<result property="name" column="sName"></result>
<association property="teacher" javaType="Teacher">
    <id column="tId" property="id"></id>  <!--id 如果不配置的话，默认返回0-->
    <result column="tName" property="name"></result>
</association>
</resultMap>

一对多　　

public class Teacher {
    private int id;
    private String name;
    private List<Student> student;

public class Student {
    private int id;
    private String name;
    private int tId;

表结构和上图一样

    <select id="getTeachers" resultMap="teacherStudentMap">
        select  t.id tId,
                t.name tName,
                s.id sId,
                s.name sName,
                s.tid sTid
         from teacher t,student s where t.id = s.tid;
    </select>
    <resultMap id="teacherStudentMap" type="Teacher">
        <result column="tId" property="id" ></result>
        <result column="tName" property="name"></result>
        <collection property="student" ofType="Student">
            <result column="sId" property="id"></result>
            <result column="sName" property="name"></result>
            <result column="sTid" property="tId"></result>
        </collection>
    </resultMap>