aRuul的无名小站

student表

teacher表

一个老师对应多个学生

Teacher.java

package org.example.pojo;

/**
 * @author aRu
 * @date 2021/3/17 20:21
 */
public class Teacher {
    private int id;
    private String name;

    public Teacher() {
    }

    public Teacher(int id, String name) {
        this.id = id;
        this.name = name;
    }

    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;
    }

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

Student.java

一个老师对应多个学生，在学生类中写上Teacher teacher

package org.example.pojo;

/**
 * @author aRu
 * @date 2021/3/17 20:20
 */
public class Student {
    private int id;
    private String name;
    private Teacher teacher;

    public Student() {
    }

    public Student(int id, String name, Teacher teacher) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
    }

    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 Teacher getTeacher() {
        return teacher;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

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

StudentMapper.java

public interface StudentMapper {
    //查询所有的学生信息 以及对应的老师的信息
    public List<Student> getStudent();
}

• 在查询学生时，学生中包含了Teacher这个类，查询的resultType比较复杂 ，所以采用resultMap的形式
• sql语句中将学生表、教师表中s.id s.name t.name字段起别名为sid sname tname

select s.id sid, s.name sname, t.name tname
        from mybatis.student s,mybatis.teacher t
        where s.tid=t.id;

• resultMap的编写

  在上一步中三个关键的数据库字段名被起别名，与原来java中的属性名不一致，所以要一一对应一下；

  对于一般的属性 用result标签

  对于类来说 用 association 标签 在java实体类中属性名property为teacher , javaType="Teacher"

  而原来java实体类中属性名name对于起别名后 的tname

StudentMapper.xml

<?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">
<!--namespace 是要绑定的类的路径-->
<mapper namespace="org.example.dao.StudentMapper">
    <!--按照结果嵌套处理-->
    <select id="getStudent" resultMap="StudentTeacher">
        select s.id sid, s.name sname, t.name tname
        from mybatis.student s,mybatis.teacher t
        where s.tid=t.id;
    </select>

    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>
</mapper>