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。

到此这篇关于Mybatis关联查询之一对多和多对一XML配置详解的文章就介绍到这了,更多相关Mybatis关联查询内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！