mybatis 一对多和多对一关联查询

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