比如：

一对多： 一个老师对应多个学生

多对一： 多个学生对应一个老师

 

建表+实体类

创建一个学生表和一个老师表

通过学生的tid与老师的id形成联系

SQLyog的建表代码

 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,'哈哈');
 ​
     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','学生1','1');
     INSERT INTO student (`id`,`name`,`tid`) VALUES ('2','学生2','1');
     INSERT INTO student (`id`,`name`,`tid`) VALUES ('3','学生3','1');
     INSERT INTO student (`id`,`name`,`tid`) VALUES ('4','学生4','1');
     INSERT INTO student (`id`,`name`,`tid`) VALUES ('5','学生5','1');

pojo（使用了Lombok）

student：

 package com.zy.pojo;
 ​
 import lombok.Data;
 ​
 @Data
 public class Student {
 ​
     private int id;
     private String name;
 ​
     private int tid;
 ​
     private Teacher teacher;
 ​
 }

teacher

 package com.zy.pojo;
 ​
 import lombok.Data;
 ​
 import java.util.List;
 ​
 @Data
 public class Teacher {
 ​
     private int id;
     private String name;
 ​
     private List<Student> students;
 ​
 }

 

多对一

多个学生对应一个老师

 

StudentMapper

 package com.zy.mapper;
 ​
 import com.zy.pojo.Student;
 ​
 import java.util.List;
 ​
 public interface StudentMapper {
 ​
     List<Student> getStudentList();
 ​
     List<Student> getStudentList2();
 ​
     List<Student> getStudentList3();
 ​
 }

StudentMapper.xml（关键点）

 <?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.zy.mapper.StudentMapper">
     <select id="getStudentList" resultType="student">
         select * from student
     </select>
 ​
 ​
     <select id="getStudentList2" resultMap="studentMap2">
         select * from student
     </select>
     <resultMap id="studentMap2" type="student">
         <result property="id" column="id"/>
         <result property="name" column="name"/>
         <result property="tid" column="tid"/>
         <association property="teacher" javaType="teacher" select="getTeacherById" column="tid"/>
     </resultMap>
     <select id="getTeacherById" resultType="teacher">
         select * from teacher where id=#{tid}
     </select>
 ​
 ​
     <select id="getStudentList3" resultMap="studentMap3">
         select s.id sid,s.name sname,s.tid stid,t.id tid,t.name tname
         from student s,teacher t
         where s.tid=t.id
     </select>
     <resultMap id="studentMap3" type="student">
         <result property="id" column="sid"/>
         <result property="name" column="sname"/>
         <result property="tid" column="stid"/>
         <association property="teacher" javaType="teacher">
             <result property="id" column="tid"/>
             <result property="name" column="tname"/>
         </association>
     </resultMap>
 </mapper>

测试

getStudentList：

 

 

 

getStudentList2：

 

getStudentList3：

 

getStudentList2与getStudentList3代码不同，效果是相同的

 

一对多

一个老师对应多个学生

TeacherMapper

 package com.zy.mapper;
 ​
 import com.zy.pojo.Teacher;
 ​
 import java.util.List;
 ​
 public interface TeacherMapper {
 ​
     List<Teacher> getTeacherList();
 ​
     List<Teacher> getTeacherList2();
 ​
     List<Teacher> getTeacherList3();
 ​
 }

TeacherMapper.xml（关键点）

 <?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.zy.mapper.TeacherMapper">
 ​
     <select id="getTeacherList" resultType="teacher">
         select * from teacher
     </select>
 ​
     <select id="getTeacherList2" resultMap="teacherMap2">
         select * from teacher
     </select>
     <resultMap id="teacherMap2" type="teacher">
         <result property="id" column="id"/>
         <result property="name" column="name"/>
         <collection property="students" javaType="ArrayList" ofType="student" select="getStudentById" column="id"/>
     </resultMap>
     <select id="getStudentById" resultType="student">
         select * from student where tid=#{id}
     </select>
     
     <select id="getTeacherList3" resultMap="teacherMap3">
         select t.id tid,t.name tname,s.id sid,s.name sname
         from teacher t,student s
         where t.id=s.tid
     </select>
     <resultMap id="teacherMap3" type="teacher">
         <result property="id" column="tid"/>
         <result property="name" column="tname"/>
         <collection property="students" javaType="ArrayList" ofType="Student">
             <result property="id" column="sid"/>
             <result property="name" column="sname"/>
             <result property="tid" column="tid"/>
         </collection>
 ​
     </resultMap>
 ​
 </mapper>

 

测试

getTeacherList：

 

getTeacherList2：

 

getTeacherList3：

 

getTeacherList2与getTeacherList3代码不同，效果是相同的

 

无论是一对多还是多对一，重点都在写xml中的代码，合理利用resultMap可以写出多个表的CRUD