Mybatis学习——一对多关联表查询

1.实体类

 public class Student {
     private int id;
     private String name;
 }
 public class Classes {
     private int id;
     private String name;
     private Teacher teacher;
     private List<Student> students;
 }

2.映射文件

 <?xml version="1.0" encoding="UTF-8" ?>
 <!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
 <mapper namespace="com.zhengbin.entity.classMapper">
     <!-- 方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集
         SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND c.c_id=1
     -->
     <select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
         select * from class c, teacher t, student s where c.teacher_id=t.t_id and c.C_id=s.class_id and c.c_id=#{id}
     </select>
     <resultMap type="Classes" id="ClassResultMap3">
         <id property="id" column="c_id" />
         <result property="name" column="c_name" />
         <association property="teacher" javaType="Teacher">
             <id property="id" column="t_id"/>
             <result property="name" column="t_name"/>
         </association>
         <!-- ofType 指定 students 集合中的对象类型 -->
         <collection property="list" ofType="Student">
             <id property="id" column="s_id" />
             <result property="name" column="s_name" />
         </collection>
     </resultMap>

     <!-- 方式二:嵌套查询:通过执行另外一个 SQL 映射语句来返回预期的复杂类型
         SELECT * FROM class WHERE c_id=1;
         SELECT * FROM teacher WHERE t_id=1
         //1 是上一个查询得到的 teacher_id 的值 SELECT*FROMstudentWHEREclass_id=1
         //1是第一个查询得到的c_id字段的值
     -->
     <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
         select * from class where c_id=#{id}
     </select>
     <resultMap type="Classes" id="ClassResultMap4">
         <id property="id" column="c_id" />
         <result property="name" column="c_name" />
         <association property="teacher" column="teacher_id" javaType="Teacher" select="getTeacher2"></association>
         <collection property="list" ofType="Student" column="c_id" select="getStudent"></collection>
     </resultMap>
     <select id="getTeacher2" parameterType="int" resultType="Teacher">
         SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
     </select>
     <select id="getStudent" parameterType="int" resultType="Student">
         SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
     </select>
 </mapper>

classMapper.xml

3.测试类

 package com.zhengbin.test;

 import java.util.List;

 import org.apache.ibatis.session.SqlSession;
 import org.apache.ibatis.session.SqlSessionFactory;

 import com.zhengbin.entity.Classes;
 import com.zhengbin.entity.Student;
 import com.zhengbin.util.MyBatisUtils;

 public class Test5 {
     @org.junit.Test
     public void getClasses(){
         SqlSessionFactory sessionFactory = MyBatisUtils.getFactory();
         // 参数为TRUE，相当于session.commit();
         SqlSession session = sessionFactory.openSession(true);
         // 读取映射文件
         String statement = "com.zhengbin.entity.classMapper" + ".getClass3";
         /*
          * 查看本班所有学生的信息，与任课老师的信息
          * 一个班级对多个学生，一个班级对一个老师
          * 即 一对多 与 一对一
          */
         Classes c = session.selectOne(statement,1);
         List<Student>list = c.getList();
         System.out.println(list);
         System.out.println(c);
         session.close();
     }

     @org.junit.Test
     public void getClasses1(){
         SqlSessionFactory sessionFactory = MyBatisUtils.getFactory();
         // 参数为TRUE，相当于session.commit();
         SqlSession session = sessionFactory.openSession(true);
         // 读取映射文件
         String statement = "com.zhengbin.entity.classMapper" + ".getClass4";
         Classes c = session.selectOne(statement,1);
         List<Student> list = c.getList();
         System.out.println(list);
         System.out.println(c);
         session.close();
     }
 }

Test.java