Mybatis 一对一、一对多、多对一

灵感来源于：https://www.cnblogs.com/xdp-gacl/p/4264440.html

转发自：https://www.cnblogs.com/hq233/p/6752335.html

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

原文地址：https://www.cnblogs.com/Bkxk/p/10175616.html