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MyBatis-多表查询(多对多)

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1、多表查询(多对多)

1.1、使用多表查询前的准备

教师表(teacher):

MyBatis- 多表查询(多对多)

教师学生表 (tors):

MyBatis- 多表查询(多对多)

1.2、需求分析

查看所有老师下面的学生信息

老师信息与学生信息为多对多的关系,一个学生有多个老师,一个老师同样可以有多个学生。要想知道老师下的学生信息,不能直接查到我们的学生信息,只能通过中间表(tors)才能关联到学生信息。

SQL 如下:

select t.tid,t.tname,s.* from teacher t left join tors ts on ts.tid=t.tid left join students s on s.sid=ts.sid order by t.tid

2、案例实现

2.1、创建 Teacher.java 文件

import java.io.Serializable; import java.util.List; public class Teacher implements Serializable {private int tid; private String tname; private List<StudentsNew> students; public int getTid() {return tid; } public void setTid(int tid) {this.tid = tid; } public String getTname() {return tname; } public void setTname(String tname) {this.tname = tname; } public List<StudentsNew> getStudents() {return students; } public void setStudents(List<StudentsNew> students) {this.students = students; } @Override public String toString() {return "Teacher{" + "tid=" + tid + ", tname='" + tname + '\'' + '}'; } }

2.2、创建 ITeacherDao.java 文件

package com.tyschool.mb005.students.dao; import com.tyschool.mb005.javabean.Teacher; import java.util.List; public interface ITeacherDao {List<Teacher> findAll(); }

2.3、创建 ITeacherDao.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.tyschool.mb005.students.dao.ITeacherDao"> <resultMap id="teacherMap" type="Teacher"> <id column="tid" property="tid"></id> <result column="tname" property="tname"></result> <collection property="students" ofType="StudentsNew"> <id column="sid" property="sid"></id> <result column="sname" property="sname"></result> <result column="ssex" property="ssex"></result> <result column="sage" property="sage"></result> </collection> </resultMap> <select id="findAll" resultMap="teacherMap"> select s.*,t.tid,t.tname from teacher t left join tors ts on ts.tid=t.tid left join students s on s.sid=ts.sid order by t.tid </select> </mapper>
<mappers> <mapper resource="com/tyschool/mb005/students/dao/ITeacherDao.xml"></mapper> </mappers>

2.4、创建 MbTeacherTest.java 文件

import com.tyschool.mb005.javabean.Classes; import com.tyschool.mb005.javabean.Teacher; import com.tyschool.mb005.students.dao.IClassesDao; import com.tyschool.mb005.students.dao.ITeacherDao; 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.After; import org.junit.Before; import org.junit.Test; import java.io.IOException; import java.io.InputStream; import java.util.List; public class MbTeacherTest {private InputStream in; private SqlSession session; private ITeacherDao teacherDao; @Test public void findAll(){List<Teacher> list=teacherDao.findAll(); for(Teacher t:list){System.out.println(t+":"+t.getStudents()); } } @Before public void init()throws IOException {in= Resources.getResourceAsStream("SqlMapConfig.xml"); SqlSessionFactoryBuilder builder=new SqlSessionFactoryBuilder(); SqlSessionFactory factory=builder.build(in); session=factory.openSession(); teacherDao=session.getMapper(ITeacherDao.class); } @After public void destroy() throws IOException {session.commit();; session.close(); in.close();} }

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