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 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 parameterType='Teacher' resultMap='teacherMaps'> SELECT t.id, t.NAME, t.class_name FROM teacher t </select> <select 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 resultMap='studentMap'> SELECT s.id, s.name, s.class_name, s.teacher_id FROM student s </select> <select 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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