Mybatis关联查询之一对多和多对一XML配置详解

平时在开发过程中dao、bean和XML文件都是自动生成的，很少写XML的配置关系，今天记录一下mybatis的关联查询中的多对一和一对多的情况。

首先是有两张表(学生表Student和老师Teacher表，注：这里只是为了演示一对多和多对一的情况，请不要杠)，为了更易懂，这里只设置了最简单的几个必要字段。表结构如下图

Student表：

Teacher表：

创建实体bean

 Teacher.java:

import java.util.List;

public class Teacher {

 private Integer id;
 private String name;
 private String className;
 private List<Student> students;

 // get、set方法省略

}

Sfudent.java

public class Student {

 private Integer id;
 private String name;
 private Integer teacherId;
 private String className;
 private Teacher teacher;
 
 // get、set方法省略
} 

下面重点来了：配置Mapper.xml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.tz.mybatis.dao.studentDao"> 
 
 <!-- ///一对多的第一种写法/// -->
 <resultMap type="Teacher" id="teacherMap">
  <id column="id" property="id"/>
  <result column="name" property="name"/>
   <collection property="students" ofType="Student" column="id">
    <!-- 这里的column对应的是下面查询的别名，而不是表字段名 -->
  <id column="sid" property="id"/>
    <!-- property对应JavaBean中的属性名 -->
  <result column="sname" property="name"/>
  <result column="className" property="className"/>
  </collection>
 </resultMap>
 
 
 <!-- 查询所有的老师级各自的所有学生 -->
 <select id="getTeachers" parameterType="Teacher" resultMap="teacherMap">
 SELECT
 t.id,
 t.NAME,
 t.class_Name,
 s.id AS sid,
 s. NAME AS sname,
 s.class_name as className
 FROM
 teacher t
 LEFT JOIN student s ON t.id = s.teacher_id
 </select>
</mapper>

测试类：

package com.tz.test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Before;
import org.junit.Test;
import com.tz.mybatis.bean.Student;
import com.tz.mybatis.bean.Teacher;

public class TeacherTest {

 private SqlSessionFactory sqlSessionFactory;
 
 @Before
 public void init() throws IOException {
 String resource = "mybatis-config.xml";
 InputStream inputStream = Resources.getResourceAsStream(resource);
 sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
 }
 
 @Test
 public void getTeachers() {
 SqlSession session = sqlSessionFactory.openSession();
 List<Teacher> list = session.selectList("com.tz.mybatis.dao.studentDao.getTeachers");
 System.out.println(list);
 }
 
}

下面给出第二种写法：

<!-- //一对多的第二种写法/ -->
 <resultMap type="Teacher" id="teacherMaps">
 <id column="id" property="id"/>
 <result column="name" property="name"/>
 <result column="class_name" property="className"/>
 <collection property="students" ofType="Student" select="getStudents" column="id">
 </collection>
 </resultMap>
 
 
 <!-- 查询所有的老师级各自的所有学生 -->
 <select id="getAllTeacher" parameterType="Teacher" resultMap="teacherMaps">
 SELECT
 t.id,
 t.NAME,
 t.class_name
 FROM
 teacher t
 </select>
 
 <select id="getStudents" parameterType="int" resultType="Student">
 select 
 s.id,
 s. NAME,
 s.class_name as className
 from student s
 where teacher_id = #{id}
 </select>

测试类：

@Test
 public void getTeachers2() {
 SqlSession session = sqlSessionFactory.openSession();
 List<Teacher> list = session.selectList("com.tz.mybatis.dao.studentDao.getAllTeacher");
 System.out.println(list);
 }

查询学生信息(多对一)：

首先还是配置文件：

<resultMap type="Student" id="studentMap">
 <id column="id" property="id"/>
 <result column="name" property="name"/>
 <result column="class_name" property="className"/>
 <result column="teacher_id" property="teacherId"/>
 <association property="teacher" select="getTeacher" column="teacher_id" javaType="Teacher">
 <!-- 这里要注意的是column对应的是student中的外键，而且需是表字段名 -->
 </association>
 </resultMap>
 
 
 <select id="getStudent" resultMap="studentMap">
 SELECT
 s.id,
 s.name,
 s.class_name,
 s.teacher_id
 FROM
 student s
 </select>
 
 <select id="getTeacher" resultType="Teacher" parameterType="int">
 SELECT
 t.id,
 t.name,
 t.class_name as className 
 FROM teacher t 
 where id = #{teacher_id}
 </select>

测试类：

@Test
 public void getStudents() {
 SqlSession session = sqlSessionFactory.openSession();
 List<Student> list = session.selectList("com.tz.mybatis.dao.studentDao.getStudent");
 System.out.println(list);
 }

最后：当然如果不想配置这么麻烦的信息，可以直接写一个关联查询的SQL语句，返回结果直接由Map接受即可。不过这样就不太符合面向对象的理念了。 

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