MyBatis 高级结果映射 | 一对多查询

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〇、需求

需求：找到指定老师对应的学生

一、搭建环境

多对一就比如多个学生对应一个老师，多对一是基于多表查询的，首先搭建MySQL环境：

CREATE DATABASE MyBatis_DB;
USE MyBatis_DB;
CREATE TABLE `teacher` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO teacher(`id`, `name`) VALUES (1, '刘老师'); 

CREATE TABLE `student` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  `tid` INT(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fktid` (`tid`),
  CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '小明', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小红', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小张', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小李', '1'); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小王', '1');

建表结果如下图所示：

打开idea，新建maven项目，然后在pom.xml文件中注入mysql、MyBatis、Junit、LOG4J依赖

然后创建两张表对应的实体类：

Teacher表对应的实体类：

package com.wzq.pojo;

import java.util.List;

public class Teacher {
    
    
    private int id;
    private String name;
    private List<Student> students;

    public int getId() {
    
    
        return id;
    }

    public void setId(int id) {
    
    
        this.id = id;
    }

    public String getName() {
    
    
        return name;
    }

    public void setName(String name) {
    
    
        this.name = name;
    }

    public List<Student> getStudents() {
    
    
        return students;
    }

    public void setStudents(List<Student> students) {
    
    
        this.students = students;
    }

    @Override
    public String toString() {
    
    
        return "Teacher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", students=" + students +
                '}';
    }
}

Student表对应的实体类：

package com.wzq.pojo;

public class Student {
    
    
    private int id;
    private String name;
    private int tid;

    public int getId() {
    
    
        return id;
    }

    public void setId(int id) {
    
    
        this.id = id;
    }

    public String getName() {
    
    
        return name;
    }

    public void setName(String name) {
    
    
        this.name = name;
    }

    public int getTid() {
    
    
        return tid;
    }

    public void setTid(int tid) {
    
    
        this.tid = tid;
    }

    @Override
    public String toString() {
    
    
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", tid=" + tid +
                '}';
    }
}

二、按照结果嵌套查询

首先在TeacherMapper中写方法：

Teacher getTeacherById1(@Param("tid") int id);

然后在TeacherMapper.xml中写sql：

    <select id="getTeacherById1" resultMap="Teacher_Student1">
        select
            t.id tid,
            t.name tname,
            s.id sid,
            s.name sname
        from
            teacher t,
            student s
        where
            t.id = s.tid and t.id = #{tid};
    </select>
    <resultMap id="Teacher_Student1" type="Teacher">
        <id property="id" column="tid" />
        <result property="name" column="tname" />
        <collection property="students" ofType="Student">
            <id property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

最后测试：

    @Test
    public void getTeacherById1Test(){
    
    
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacherById1(1);
        System.out.println(teacher);
        sqlSession.close();
    }

结果：

[com.wzq.dao.TeacherMapper.getTeacherById1]-==>  Preparing: select t.id tid, t.name tname, s.id sid, s.name sname from teacher t, student s where t.id = s.tid and t.id = ?;
[com.wzq.dao.TeacherMapper.getTeacherById1]-==> Parameters: 1(Integer)
[com.wzq.dao.TeacherMapper.getTeacherById1]-<==      Total: 5
Teacher{
    
    id=1, name='刘老师', students=[Student{
    
    id=1, name='小明', tid=1}, Student{
    
    id=2, name='小红', tid=1}, Student{
    
    id=3, name='小张', tid=1}, Student{
    
    id=4, name='小李', tid=1}, Student{
    
    id=5, name='小王', tid=1}]}

三、按照查询嵌套处理

同上面的步骤，写方法：

Teacher getTeacherById2(@Param("tid") int id);

然后在TeacherMapper.xml中写sql：

    <select id="getTeacherById2" resultMap="Teacher_Student2">
        select * from teacher where id = #{tid};
    </select>
    <resultMap id="Teacher_Student2" type="Teacher">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="students" javaType="ArrayList" ofType="Students" select="getStudentByTeacherId" column="id"/>
    </resultMap>
    <select id="getStudentByTeacherId" resultType="Student">
        select * from student where tid = #{tid};
    </select>

最后测试：

    @Test
    public void getTeacherById2Test(){
    
    
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacherById2(1);
        System.out.println(teacher);
        sqlSession.close();
    }