mybatis复杂查询环境 多对一的处理 按照结果嵌套处理和按照查询嵌套处理

sql插入表student的语句：

insert into student (id, name, tid) values ('9', '梁梁','2');
insert into student (id, name,tid) values ('7', '亮亮','2');

两张表：




所对应的实体类，学术类：

package com.kuang.pojo;

import lombok.Data;


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

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

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

    private Teacher teacher;//利用这个来做到一个老师关联多个学生
}

老师:

package com.kuang.pojo;

import lombok.Data;

import java.util.List;

@Data
public class Teacher {
    
    
    private int id;
    private String name;
private List<Student>stus;//一个老师关联多个学生
}

多对一查询语句：
sql编辑器下面输入：

select s.id,s.name,t.name from student s,teacher t where s.tid = t.id;

显示:

嵌套查询：

按照查询嵌套处理

目录：

查询所有的学生信息
根据查询出来的学生tid,寻找对应的老师，子查询：

<mapper namespace="com.kuang.dao.Studenmapper">
<select id="getstudent" resultMap="StudenTeacher">
    select * from student
</select>
    <resultMap id="StudenTeacher" type="com.kuang.pojo.Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <association property="teacher" column="tid" javaType="com.kuang.pojo.Teacher" select="getTeacher"/>
    </resultMap>
<select id="getTeacher" resultType="com.kuang.pojo.Teacher">
    select * from teacher where id=#{id}
</select>
</mapper>

复杂的属性，我们需要单独处理，对象：association,集合:collection
测试类：

   @Test
    public void  testStudent(){
    
    
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        Studenmapper stuma = sqlSession.getMapper(Studenmapper.class);
        List<Student> stu = stuma.getstudent();
        for(Student stu1:stu){
    
    
            System.out.println(stu1);
        }
        sqlSession.close();
    }

注意，type要用全类名，不然会报错

按照结果嵌套处理

测试类：

  @Test
    public void  testStudent(){
    
    
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        Studenmapper stuma = sqlSession.getMapper(Studenmapper.class);
        List<Student> stu = stuma.getstudent2();
        for(Student stu1:stu){
    
    
            System.out.println(stu1);
        }
        sqlSession.close();
    }

Studenmapper:

package com.kuang.dao;
import com.kuang.pojo.Student;

import java.util.List;

public interface Studenmapper
{
    
    
    public List<Student>getstudent();
    public List<Student>getstudent2();
}

xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.kuang.dao.Studenmapper">
<select id="getstudent2" resultMap="StudenTeacher2">
    select s.id sid,s.name sname,t.name tname
    from student s,teacher t
    where s.tid=t.id;
</select>
    <resultMap id="StudenTeacher2" type="com.kuang.pojo.Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher"  javaType="com.kuang.pojo.Teacher" >
                <result property="name" column="tname"></result>
        </association>
    </resultMap>

</mapper>

这里因为把teacher拼成了teahcer导致报错，还有注意type的写法是全类名。
select s.id sid,s.name sname,t.name tname：
取了三个别名