Mybatis 一对多和多对一关联查询问题

首先  数据库量表之间字段关系（没有主外键）

studentmajor表的id字段对应student表里major字段

两个实体类

package com.model;
import java.util.date;
public class student {
  private integer sno;
  private string sname;
  private string ssex;
  private integer sclass;
  private studentmajor studentmajor;
  public student() {
    super();
  }
  public student(integer sno, string sname, string ssex, integer sclass, studentmajor studentmajor) {
    super();
    this.sno = sno;
    this.sname = sname;
    this.ssex = ssex;
    this.sclass = sclass;
    this.studentmajor = studentmajor;
  }
  public studentmajor getstudentmajor() {
    return studentmajor;
  }
  public void setstudentmajor(studentmajor studentmajor) {
    this.studentmajor = studentmajor;
  }
  public integer getsno() {
    return sno;
  }
  public void setsno(integer sno) {
    this.sno = sno;
  }
  public string getsname() {
    return sname;
  }
  public void setsname(string sname) {
    this.sname = sname;
  }
  public string getssex() {
    return ssex;
  }
  public void setssex(string ssex) {
    this.ssex = ssex;
  }
  @override
  public string tostring() {
    return "student [sno=" + sno + ", sname=" + sname + ", ssex=" + ssex + ", sclass=" + sclass + ", studentmajor="
        + studentmajor + "]";
  }
  public integer getsclass() {
    return sclass;
  }
  public void setsclass(integer sclass) {
    this.sclass = sclass;
  }
}

package com.model;
import java.util.list;
public class studentmajor {
  private integer id;
  private string mcode;
  private string mname;
  private list<student> students;
  public studentmajor() {
    super();
  }
  public studentmajor(integer id, string mcode, string mname, list<student> students) {
    super();
    this.id = id;
    this.mcode = mcode;
    this.mname = mname;
    this.students = students;
  }
  @override
  public string tostring() {
    return "studentmajor [id=" + id + ", mcode=" + mcode + ", mname=" + mname + ", students=" + students + "]";
  }
  public integer getid() {
    return id;
  }
  public void setid(integer id) {
    this.id = id;
  }
  public string getmcode() {
    return mcode;
  }
  public void setmcode(string mcode) {
    this.mcode = mcode;
  }
  public string getmname() {
    return mname;
  }
  public void setmname(string mname) {
    this.mname = mname;
  }
  public list<student> getstudents() {
    return students;
  }
  public void setstudents(list<student> students) {
    this.students = students;
  }
}

定义两个接口

package com.dao;
import java.util.list;
import java.util.map;
import com.model.student;
public interface studentmapper {
  /**
   * 全表查询
   */
  public list<student> selectall();
　　/**
   * 根据专业查人员,给一对多用
   */
  public list<student> selectz(integer major);
}

package com.dao;
import java.util.list;
import com.model.studentmajor;
public interface studentmajormapper {
  /**
   * 全表查询
   * @return
   */
  public list<studentmajor> selectall();
  /**
   * 根据主键查数据,给多对一用
   * @param id
   * @return
   */
  public studentmajor select(integer id);
}

定义两个实体类的映射方法

<?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.dao.studentmapper">
  <!-- 多对一查询 -->
  <resultmap type="student" id="slist">
    <!-- 跟一对一一样用association标签,实体类定义的成员,要跟数据库字段名对应上 -->
    <association property="studentmajor" column="major" 
    select="com.dao.studentmajormapper.select"/> <!-- 用接口里定义的方法，根据student表中的major字段查出对应数据 -->
  </resultmap>
  <!-- 查全部 -->
  <select id="selectall" resultmap="slist" >
    select * from student
  </select>
  <!-- 根据专业查人员 -->
  <select id="selectz" parametertype="integer" resulttype="student">
    select * from student s where s.major=#{major}
  </select>
 </mapper>

<?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.dao.studentmajormapper">
   <!-- 一对多查询关联 -->
   <resultmap type="studentmajor" id="slist">
     <!-- 实体类属性对应数据库的主键字段，不然主键会查不到 -->
     <id property="id" column="id"/>
     <!-- 用collection标签 ,也是实体类属性要对应数据库字段-->
     <collection property="students" column="id"
     select="com.dao.studentmapper.selectz">
     </collection>  
   </resultmap>
   <!-- 全表查询 -->
   <select id="selectall" resultmap="slist">
     select * from studentmajor
   </select>
   <!-- 根据主键查 -->
   <select id="select" parametertype="integer" resulttype="studentmajor">
     select * from studentmajor where id=#{id}
   </select>
 </mapper>

junit测试

package com.util;
import java.util.list;
import org.apache.ibatis.session.sqlsession;
import org.junit.after;
import org.junit.before;
import org.junit.test;
import com.dao.studentmajormapper;
import com.dao.studentmapper;
import com.model.student;
import com.model.studentmajor;
public class jjjtest {
  private sqlsession ss;
  private studentmapper sm;
  private studentmajormapper smm;
  @before
  public void setup() throws exception {
    ss=sqlsessionutil.getsqlsession();
    sm=ss.getmapper(studentmapper.class);
    smm=ss.getmapper(studentmajormapper.class);
  }
  @after
  public void teardown() throws exception {
    ss.commit();
    ss.close();
  }
  //一对多查询
  public void test() {
    list<studentmajor> list=smm.selectall();
    for(studentmajor a:list){
      system.out.println(a);
    }
  }
  //根据专业查人员，给一对多用
  public void selectz(){
    list<student> l=sm.selectz(3);
    for(student a:l){
      system.out.println(a);
    }
  }
  //多对一查询
  @test
  public void selectall() {
    list<student> st=sm.selectall();
    for(student tt:st){
      system.out.println(tt);
    }
  }
  //根据主键查询，给多对一用
  public void select(){
    studentmajor a=smm.select(1);
    system.out.println(a);
  }
}

一对多查询结果

多对一查询结果

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