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
一、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;
}
二、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
三、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
比如 Mybatis官网 的例子：
<!-- 非常复杂的语句 --> <select id="selectBlogDetails" resultMap="detailedBlogResultMap"> select B.id as blog_id, B.title as blog_title, B.author_id as blog_author_id, A.id as author_id, A.username as author_username, A.password as author_password, A.email as author_email, A.bio as author_bio, A.favourite_section as author_favourite_section, P.id as post_id, P.blog_id as post_blog_id, P.author_id as post_author_id, P.created_on as post_created_on, P.section as post_section, P.subject as post_subject, P.draft as draft, P.body as post_body, C.id as comment_id, C.post_id as comment_post_id, C.name as comment_name, C.comment as comment_text, T.id as tag_id, T.name as tag_name from Blog B left outer join Author A on B.author_id = A.id left outer join Post P on B.id = P.blog_id left outer join Comment C on P.id = C.post_id left outer join Post_Tag PT on PT.post_id = P.id left outer join Tag T on PT.tag_id = T.id where B.id = #{id} </select>