Mybatis关联查询之一对多和多对一XML配置详解
https://blog.csdn.net/qq_33561055/article/details/78861131
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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; } }
Sfudent.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>
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。
