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)，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;

 /**
  * TODO
  * @version 创建时间：2017年12月21日 上午9:02:45
  */
 public class Teacher {

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

     public List<Student> getStudents() {
         return students;
     }

     public void setStudents(List<Student> students) {
         this.students = students;
     }

     public Integer getId() {
         return id;
     }

     public void setId(Integer id) {
         this.id = id;
     }

     public String getName() {
         return name;
     }

     public void setName(String name) {
         this.name = name;
     }

     public String getClassName() {
         return className;
     }

     public void setClassName(String className) {
         this.className = className;
     }

 }

Student.java

/**
 * TODO
 *
 * @author 作者 E-mail:2332999366@qq.com
 * @version 创建时间：2017年12月21日 上午9:01:17
 */
public class Student {

    private Integer id;
    private String name;
    private Integer teacherId;
    private String className;
    private Teacher teacher;

    public Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getTeacherId() {
        return teacherId;
    }

    public void setTeacherId(Integer teacherId) {
        this.teacherId = teacherId;
    }

    public String getClassName() {
        return className;
    }

    public void setClassName(String className) {
        this.className = className;
    }

    @Override
    public String toString() {
        return "{id:"+this.id+",name:"+this.name+",className:"+this.className+",teacherId:"+this.teacherId+"}";
    }
} 

　　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，一般考虑到性能问题，不会这么实现//////////////////////// -->
    <resultMap type="Teacher" id="teacherMap">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <collection property="students" ofType="Student" column="id">
            <id column="sid" property="id"/><!-- 这里的column对应的是下面查询的别名，而不是表字段名 -->
            <result column="sname" property="name"/><!-- property对应JavaBean中的属性名 -->
            <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>

请注意看注释，很重要哦~

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。