Mybatis一对多和多对一的处理

程序员文章站 2022-06-22 17:16:47

文章目录一、多对一的处理1.数据库建表2.student和teacher的实体类3.StudentMapper接口4.StudentMapper.xml文件配置二、一对多的处理一、多对一的处理多位student对应一位teacher，我们需要实现的操作是，查询同学的同时也能查到对应的老师。1.数据库建表teacher表CREATE TABLE `teacher` (`id` int(15) NOT NULL ,`name` varchar(25),PRIMARY KEY (`id...

文章目录

一、多对一的处理
二、一对多的处理
小结

一、多对一的处理

多位student对应一位teacher，我们需要实现的操作是，查询同学的同时也能查到对应的老师。
Mybatis一对多和多对一的处理

1.数据库建表

teacher表

CREATE TABLE `teacher` (
`id`  int(15) NOT NULL ,
`name`  varchar(25),
PRIMARY KEY (`id`)
);

student表

CREATE TABLE `student` (
`id`  int(15) NOT NULL ,
`name`  varchar(25),
`tid`  int(15) NOT NULL ,
PRIMARY KEY (`id`),
CONSTRAINT `tid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT
)
;

2.student和teacher的实体类

Student.java

package dao.pojo;

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 +
                '}';
    }
}

Teacher.java

package dao.pojo;

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 + '\'' +
                '}';
    }
}

3.StudentMapper接口

我们的操作暂时在student上，不需要Teacher接口。

package dao.userdao;

import dao.pojo.Student;

import java.util.List;

public interface StudentMapper {
    
    List<Student> getStudents();
}

4.StudentMapper.xml文件配置

原来的查询teacher查出来是null

<?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="dao.userdao.StudentMapper">

    <select id="getStudents" resultType="student">
    select * from test_student
    </select>

</mapper>

Mybatis一对多和多对一的处理

两种解决办法：按照查询嵌套处理和按照结果嵌套处理
按照查询嵌套处理:

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">

<mapper namespace="dao.userdao.StudentMapper">

    <!--
    思路：1.查询所有的学生信息
         2.根据查询出来的学生的tid,寻找对应的老师
    -->
    
    <select id="getStudents" resultMap="StudentTeacher">
        select * from test_student
    </select>

    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--复杂的属性，我们需要单独处理  对象：association  集合：collection-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>

    <select id="getTeacher" resultType="Teacher">
        select * from test_teacher where id=#{id}
    </select>

</mapper>

Mybatis一对多和多对一的处理

按照结果嵌套处理：

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">

<mapper namespace="dao.userdao.StudentMapper">

    <select id="getStudents" resultMap="StudentTeacher">
        select s.id sid,s.name sname,t.id stid,t.name tname
        from test_student s,test_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" column="tname" javaType="Teacher">
            <result property="id" column="stid"/>
            <result property="name" column="tname"/>
        </association>
    </resultMap>

</mapper>

Mybatis一对多和多对一的处理

二、一对多的处理

一位teacher管理多位student，我们需要实现的操作是，查询老师的同时也能查到对应的所有学生。

Mybatis一对多和多对一的处理

1.student和teacher的实体类

Student.java

package dao.pojo;

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

    public Student() {
    }

    public Student(int id, String name, int tid) {
        this.id = id;
        this.name = name;
        this.tid = 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 +
                '}';
    }
}

Teacher.java

package dao.pojo;

import java.util.List;

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

    public Teacher() {
    }

    public Teacher(int id, String name, List<Student> students) {
        this.id = id;
        this.name = name;
        this.students = 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 +
                '}';
    }
}

2.TeacherMapper接口

package dao.userdao;


import dao.pojo.Teacher;
import org.apache.ibatis.annotations.Param;

public interface TeacherMapper {

    //根据老师的id，查询老师和他管理的学生
    Teacher getTeacherById(@Param("id") int id);
}

3.TeacherMapper.xml文件配置

按照结果嵌套处理:

TeacherMapper.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">

<mapper namespace="dao.userdao.TeacherMapper">

    <select id="getTeacherById" resultMap="TeacherStudent">
        select s.id sid,s.name sname,t.id tid,t.name tname
        from test_student s,test_teacher t 
        where s.tid=t.id and t.id=#{id}
    </select>
    
    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!--
             复杂的属性，我们需要单独处理   对象： association   集合：collection
             javaType=""  指定属性的类型
             集合中的泛型信息，我们使用ofType获取
        -->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

</mapper>

Mybatis一对多和多对一的处理

按照查询嵌套处理

TeacherMapper.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">

<mapper namespace="dao.userdao.TeacherMapper">

    <select id="getTeacherById" resultMap="TeacherStudent">
        select * from test_teacher where id=#{id}
    </select>

    <resultMap id="TeacherStudent" type="Teacher">
        <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentsByTeacherId" column="id"/>
    </resultMap>

    <select id="getStudentsByTeacherId" resultType="student" >
        select * from test_student where tid=#{id}
    </select>
    


</mapper>

Mybatis一对多和多对一的处理

小结

关联-association （多对一）
集合-collection （一对多）
javaType & ofType

javaType：用来指定实体类中属性的类型
ofType：用来指定映射到List或集合中的pojo类型，泛型中的约束型

本文地址：https://blog.csdn.net/qq_43466788/article/details/108568065

上一篇： SQL Server 2008及更高版本数据库恢复方法之日志尾部备份

下一篇：太喜欢三伏天了

Mybatis一对多和多对一的处理

文章目录

一、多对一的处理

1.数据库建表

2.student和teacher的实体类

3.StudentMapper接口

4.StudentMapper.xml文件配置

二、一对多的处理

1.student和teacher的实体类

2.TeacherMapper接口

3.TeacherMapper.xml文件配置

小结

MyBatis从入门到精通(十一)：MyBatis高级结果映射之一对多映射

EXCEL VLOOKUP一对多查找根据部门查找员工姓名

JPA中实现双向一对多的关联关系

关于MyBatis 查询数据时属性中多对一的问题(多条数据对应一条数据)

mybatis 多对一关联查询

Django 标签筛选的实现代码(一对多、多对多)

牛奶和芒果可以一起吃吗？原来吃它的好处有这么多！

金针菜和黄花菜一样吗，吃黄花菜的好处有这么多

JPA中实现单向一对多的关联关系

Flask SQLAlchemy一对一,一对多的使用方法实践

Mybatis一对多和多对一的处理

文章目录

一、多对一的处理

1.数据库建表

2.student和teacher的实体类

3.StudentMapper接口

4.StudentMapper.xml文件配置

二、一对多的处理

1.student和teacher的实体类

2.TeacherMapper接口

3.TeacherMapper.xml文件配置

小结

MyBatis从入门到精通(十一)：MyBatis高级结果映射之一对多映射

EXCEL VLOOKUP一对多查找 根据部门查找员工姓名

JPA中实现双向一对多的关联关系

关于MyBatis 查询数据时属性中多对一的问题(多条数据对应一条数据)

mybatis 多对一关联查询

Django 标签筛选的实现代码(一对多、多对多)

牛奶和芒果可以一起吃吗？原来吃它的好处有这么多！

金针菜和黄花菜一样吗，吃黄花菜的好处有这么多

JPA中实现单向一对多的关联关系

Flask SQLAlchemy一对一,一对多的使用方法实践

EXCEL VLOOKUP一对多查找根据部门查找员工姓名