java中一对多查询_一对多+多对一关系的查询
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：
packagecom.zy.pojo;
importlombok.Data;
@Datapublic classStudent {
private intid;privateString name;
private inttid;
privateTeacher teacher;
}
teacher
packagecom.zy.pojo;
importlombok.Data;
importjava.util.List;
@Datapublic classTeacher {
private intid;privateString name;
private Liststudents;
}
二、多对一
多个学生对应一个老师
StudentMapper
packagecom.zy.mapper;
importcom.zy.pojo.Student;
importjava.util.List;
public interfaceStudentMapper {
ListgetStudentList();
ListgetStudentList2();
ListgetStudentList3();
}
StudentMapper.xml(关键点)
/p>
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
select * from student
select * from student
select * from teacher where id=#{tid}
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
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
packagecom.zy.mapper;
importcom.zy.pojo.Teacher;
importjava.util.List;
public interfaceTeacherMapper {
ListgetTeacherList();
ListgetTeacherList2();
ListgetTeacherList3();
}
TeacherMapper.xml(关键点)
/p>
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
select * from teacherselect * from teacher
select * from student where tid=#{id}
select t.id tid,t.name tname,s.id sid,s.name sname
from teacher t,student s
where t.id=s.tid
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
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}