注:代码已托管在GitHub
上,地址是:https://github.com/Damaer/Mybatis-Learning
,项目是mybatis-15-oneself-many2many
,需要自取,需要配置maven环境以及mysql
环境(sql
语句在resource
下的test.sql
中),觉得有用可以点个小星星。
docsify
文档地址在:https://damaer.github.io/Mybatis-Learning/#/
所谓多对多查询,就是类似于:一个学生可以选多门课程,一门可能可以有多个学生。
数据表设计如下:
与数据库对应的实体类Course.java
,值得注意的是,toString()
方法里面我们没有加入students
属性,这是因为在Student
的tostring()
方法里面已经加入我们的Course
这个类了,如果这里加入就会死循环,只加一个就可以了。
import java.util.Set;
public class Course {
private Integer cid;
private String cname;
private Set<Student>students;
@Override
public String toString() {
return "Course [cid=" + cid + ", cname=" + cname +"]";
}
public Integer getCid() {
return cid;
}
public void setCid(Integer cid) {
this.cid = cid;
}
public String getCname() {
return cname;
}
public void setCname(String cname) {
this.cname = cname;
}
public Set<Student> getStudents() {
return students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}
}
Student.java
:
import java.util.Set;
public class Student {
private Integer sid;
private String sname;
private Set<Course>courses;
public Integer getSid() {
return sid;
}
public void setSid(Integer sid) {
this.sid = sid;
}
public String getSname() {
return sname;
}
public void setSname(String sname) {
this.sname = sname;
}
public Set<Course> getCourses() {
return courses;
}
public void setCourses(Set<Course> courses) {
this.courses = courses;
}
@Override
public String toString() {
return "Student [sid=" + sid + ", sname=" + sname + ", courses="
+ courses + "]";
}
}
定义的接口部分:
public interface IStudentDao {
Student selectStudentById(int id);
}
mapper.xml
文件,查询的时候,查的是三张表,通过sid=studentId and cid=courseId and sid=#{xxx}
关联起来。
对结果做了一个映射,除了主键以及sname
,courses
属性做了集合映射,也就是对Course
类型进行映射。
<?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.IStudentDao">
<resultMap type="Student" id="studentMapper">
<id column="sid" property="sid"/>
<result column="sname" property="sname"/>
<collection property="courses" ofType="Course">
<id column="cid" property="cid"/>
<result column="cname" property="cname"/>
</collection>
</resultMap>
<select id="selectStudentById" resultMap="studentMapper">
select sid,sname,cid,cname
from student,middle,course
where sid=studentId and cid=courseId and sid=#{xxx}
</select>
</mapper>
单元测试类:
public class MyTest {
private IStudentDao dao;
private SqlSession sqlSession;
@Before
public void Before(){
sqlSession=MyBatisUtils.getSqlSession();
dao=sqlSession.getMapper(IStudentDao.class);
}
@Test
public void TestselectMinisterById(){
Student student=dao.selectStudentById(1);
System.out.println(student);
}
@After
public void after(){
if(sqlSession!=null){
sqlSession.close();
}
}
}
结果:
[service] 2018-07-16 20:25:37,846 - dao.IStudentDao.selectStudentById -843 [main] DEBUG dao.IStudentDao.selectStudentById - ==> Preparing: select sid,sname,cid,cname from student,middle,course where sid=studentId and cid=courseId and sid=?
[service] 2018-07-16 20:25:37,894 - dao.IStudentDao.selectStudentById -891 [main] DEBUG dao.IStudentDao.selectStudentById - ==> Parameters: 1(Integer)
[service] 2018-07-16 20:25:37,935 - dao.IStudentDao.selectStudentById -932 [main] DEBUG dao.IStudentDao.selectStudentById - <== Total: 2
Student [sid=1, sname=Jam, courses=[Course [cid=1, cname=JAVA], Course [cid=2, cname=C++]]]
【作者简介】:
秦怀,公众号【秦怀杂货店】作者,技术之路不在一时,山高水长,纵使缓慢,驰而不息。个人写作方向:Java源码解析,JDBC,Mybatis,Spring,redis,分布式,剑指Offer,LeetCode等,认真写好每一篇文章,不喜欢标题党,不喜欢花里胡哨,大多写系列文章,不能保证我写的都完全正确,但是我保证所写的均经过实践或者查找资料。遗漏或者错误之处,还望指正。
平日时间宝贵,只能使用晚上以及周末时间学习写作,关注我,我们一起成长吧~