Mybatis【一对多、多对一、多对多】知识要点
Mybatis【多表连接】
我们在学习Hibernate的时候，如果表涉及到两张的话，那么我们是在映射文件中使用<set>..<many-to-one>等标签将其的映射属性关联起来的...那么在我们Mybatis中又怎么做呢？？？
先来回顾一下我们SQL99的语法：
一）内连接（等值连接）：查询客户姓名，订单编号，订单价格
---------------------------------------------------
select c.name,o.isbn,o.price
from customers c inner join orders o
where c.id = o.customers_id;
---------------------------------------------------
select c.name,o.isbn,o.price
from customers c join orders o
where c.id = o.customers_id;
---------------------------------------------------
select c.name,o.isbn,o.price
from customers c,orders o
where c.id = o.customers_id;
---------------------------------------------------
select c.name,o.isbn,o.price
from customers c join orders o
on c.id = o.customers_id;
---------------------------------------------------
注意：内连接（等值连接）只能查询出多张表中，连接字段相同的记录
二）外连接：按客户分组，查询每个客户的姓名和订单数
---------------------------------------------------
左外连接：
select c.name,count(o.isbn)
from customers c left outer join orders o
on c.id = o.customers_id
group by c.name;
---------------------------------------------------
右外连接：
select c.name,count(o.isbn)
from orders o right outer join customers c
on c.id = o.customers_id
group by c.name;
---------------------------------------------------
注意：外连接既能查询出多张表中，连接字段相同的记录；又能根据一方，将另一方不符合相同记录强行查询出来
三）自连接：求出AA的老板是EE
---------------------------------------------------
内自连接：
select users.ename,boss.ename
from emps users inner join emps boss
on users.mgr = boss.empno;
---------------------------------------------------
外自连接：
select users.ename,boss.ename
from emps users left outer join emps boss
on users.mgr = boss.empno;
---------------------------------------------------
注意：自连接是将一张表，通过别名的方式，看作多张表后，再进行连接。
这时的连接即可以采用内连接，又可以采用外连接由于我们Mybatis中并没有像Hibernate这样全自动化的，因此我们是没有<set>..<many-to-one>等标签的，我们还是使用手写SQL语句来使我们的关联属性连接起来...
一对一
需求：
- 学生和身份证
设计表：
--mysql
create table cards(
cid int(5) primary key,
cnum varchar(10)
);
create table students(
sid int(5) primary key,
sname varchar(10),
scid int(5),
constraint scid_fk foreign key(scid) references cards(cid)
);
insert into cards(cid,cnum) values(1,'111');
insert into students(sid,sname,scid) values(1,'哈哈',1);
select * from cards;
select * from students;实体
/**
* 身份证(单方)
* @author AdminTC
*/
public class Card {
private Integer id;
private String num;
public Card(){}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getNum() {
return num;
}
public void setNum(String num) {
this.num = num;
}
}/**
* 学生(单方)
* @author AdminTC
*/
public class Student {
private Integer id;
private String name;
private Card card;//关联属性
public Student(){}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Card getCard() {
return card;
}
public void setCard(Card card) {
this.card = card;
}
}映射文件
由于我们有两个实体，因此我们会有两个映射文件
Student映射文件
<?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="studentNamespace">
<resultMap type="zhongfucheng2.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
</resultMap>
</mapper>Card映射文件
<?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="cardNamespace">
<resultMap type="zhongfucheng2.Card" id="cardMap">
<id property="id" column="cid"/>
<result property="num" column="cnum"/>
</resultMap>
</mapper>DAO层
现在我想根据学生的编号查询学生的信息和身份证信息！
由于该查询着重是查询学生的信息，于是我们在学生的映射文件中写SQL语句
按照需求，我们写出来的SQL语句是这样子的。
select * from zhongfucheng.students s,zhongfucheng.cards c where c.cid = s.scid and sid=;
我来看一下查询结果：
我们的实体与映射表中，Student实体是没有关联其他的字段的，仅仅是写出了该实体的自带的属性。
<resultMap type="zhongfucheng2.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
</resultMap>明显地，我们Student是不能封装返回的结果，因此我们需要将关联属性进行关联起来！
<resultMap type="zhongfucheng2.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
<!--
property写的是在Student实体中写关联字段的属性变量名称
resultMap写的是映射文件中的命名空间.id
-->
<association property="card" resultMap="cardNamespace.cardMap"/>
</resultMap>我们关联了以后，Student实体就能够封装返回的结果了
<resultMap type="zhongfucheng2.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
<!--
property写的是在Student实体中写关联字段的属性变量名称
resultMap写的是映射文件中的命名空间.id
-->
<association property="card" resultMap="cardNamespace.cardMap"/>
</resultMap>
<select id="findById" parameterType="int" resultMap="studentMap">
select * from zhongfucheng.students s,zhongfucheng.cards c where c.cid = s.scid and sid=#{id};
</select>查询编号为1的学生信息【包括身份证编号】
public Student findById(int id) throws Exception {
//得到连接对象
SqlSession sqlSession = MybatisUtil.getSqlSession();
try{
return sqlSession.selectOne("studentNamespace.findById", id);
/* sqlSession.commit();*/
}catch(Exception e){
e.printStackTrace();
sqlSession.rollback();
throw e;
}finally{
MybatisUtil.closeSqlSession();
}
}
public static void main(String[] args) throws Exception {
StudentDao studentDao = new StudentDao();
Student student = studentDao.findById();
System.out.println(student.getId() + "----" + student.getName() + "----" + student.getCard().getNum());
}
一对多
需求：
- 一个班级有多个学生,查询java学科有哪些学生信息
设计数据库表
create table grades( gid int(5) primary key, gname varchar(10) ); create table students( sid int(5) primary key, sname varchar(10), sgid int(5), constraint sgid_fk foreign key(sgid) references grades(gid) ); insert into grades(gid,gname) values(1,'java'); insert into students(sid,sname,sgid) values(1,'哈哈',1); insert into students(sid,sname,sgid) values(2,'呵呵',1); select * from grades; select * from students;
实体
package zhongfucheng2;
import java.util.ArrayList;
import java.util.List;
/**
* 学科(单方)
* @author AdminTC
*/
public class Grade {
private Integer id;
private String name;
private List<Student> studentList = new ArrayList<Student>();//关联属性
public Grade(){}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Student> getStudentList() {
return studentList;
}
public void setStudentList(List<Student> studentList) {
this.studentList = studentList;
}
}package zhongfucheng2;
/**
* 学生(多方)
* @author AdminTC
*/
public class Student {
private Integer id;
private String name;
private Grade grade;//关联属性
public Student(){}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Grade getGrade() {
return grade;
}
public void setGrade(Grade grade) {
this.grade = grade;
}
}映射文件SQL语句
<mapper namespace="studentNamespace">
<resultMap type="zhongfucheng2.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
</resultMap>
<!--查询选修的java学科有多少位学生-->
<!--由于我们只要查询学生的名字，而我们的实体studentMap可以封装学生的名字，那么我们返回studentMap即可，并不需要再关联到学科表-->
<select id="findByGrade" parameterType="string" resultMap="studentMap">
select s.sname,s.sid from zhongfucheng.students s,zhongfucheng.grades g WHERE s.sgid=g.gid and g.gname=#{name};
</select>
</mapper>
<?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="gradeNamespace">
<resultMap type="zhongfucheng2.Grade" id="gradeMap">
<id property="id" column="gid"/>
<result property="name" column="gname"/>
</resultMap>
</mapper>DAO
public List<Student> findByGrade(String grade) throws Exception {
//得到连接对象
SqlSession sqlSession = MybatisUtil.getSqlSession();
try{
return sqlSession.selectList("studentNamespace.findByGrade", grade);
/* sqlSession.commit();*/
}catch(Exception e){
e.printStackTrace();
sqlSession.rollback();
throw e;
}finally{
MybatisUtil.closeSqlSession();
}
}
public static void main(String[] args) throws Exception {
StudentDao studentDao = new StudentDao();
List<Student> student = studentDao.findByGrade("java");
for (Student student1 : student) {
System.out.println(student1.getName());
}
}
多对多
需求：
- 学生和课程
数据库表
create table students(
sid int(5) primary key,
sname varchar(10)
);
create table courses(
cid int(5) primary key,
cname varchar(10)
);
create table middles(
msid int(5),
mcid int(5),
primary key(msid,mcid)
);
insert into students(sid,sname) values(1,'哈哈');
insert into students(sid,sname) values(2,'呵呵');
insert into courses(cid,cname) values(1,'java');
insert into courses(cid,cname) values(2,'android');
insert into middles(msid,mcid) values(1,1);
insert into middles(msid,mcid) values(1,2);
insert into middles(msid,mcid) values(2,1);
insert into middles(msid,mcid) values(2,2);
select * from students;
select * from courses;
select * from middles;实体
package cn.itcast.javaee.mybatis.many2many;
import java.util.ArrayList;
import java.util.List;
/**
* 课程(多方)
* @author AdminTC
*/
public class Course {
private Integer id;
private String name;
private List<Student> studentList = new ArrayList<Student>();//关联属性
public Course(){}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Student> getStudentList() {
return studentList;
}
public void setStudentList(List<Student> studentList) {
this.studentList = studentList;
}
}package cn.itcast.javaee.mybatis.many2many;
import java.util.ArrayList;
import java.util.List;
/**
* 学生(多方)
* @author AdminTC
*/
public class Student {
private Integer id;
private String name;
private List<Course> courseList = new ArrayList<Course>();//关联属性
public Student(){}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<Course> getCourseList() {
return courseList;
}
public void setCourseList(List<Course> courseList) {
this.courseList = courseList;
}
}映射文件
<?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="courseNamespace">
<resultMap type="cn.itcast.javaee.mybatis.many2many.Course" id="courseMap">
<id property="id" column="cid"/>
<result property="name" column="cname"/>
</resultMap>
<!-- 查询哈哈选学了哪些课程 -->
<select id="findAllByName" parameterType="string" resultMap="courseMap">
select c.cid,c.cname
from students s inner join middles m
on s.sid = m.msid
inner join courses c
on m.mcid = c.cid
and s.sname = #{name}
</select>
</mapper><?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="studentNamespace">
<resultMap type="cn.itcast.javaee.mybatis.many2many.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
</resultMap>
<select id="findAllByCourseName" parameterType="string" resultMap="studentMap">
select s.sname
from students s inner join middles m
on s.sid = m.msid
inner join courses c
on m.mcid = c.cid
and c.cname = #{name}
</select>
</mapper>DAO
package cn.itcast.javaee.mybatis.many2many;
import java.util.List;
import org.apache.ibatis.session.SqlSession;
import cn.itcast.javaee.mybatis.util.MybatisUtil;
/**
* 持久层
* @author AdminTC
*/
public class StudentCourseDao {
/**
* 查询哈哈选学了哪些课程
* @param name 表示学生的姓名
*/
public List<Course> findAllByName(String name) throws Exception{
SqlSession sqlSession = null;
try{
sqlSession = MybatisUtil.getSqlSession();
return sqlSession.selectList("courseNamespace.findAllByName",name);
}catch(Exception e){
e.printStackTrace();
throw e;
}finally{
MybatisUtil.closeSqlSession();
}
}
/**
* 查询java课程有哪些学生选修
* @param name 表示学生的课程
*/
public List<Student> findAllByCourseName(String name) throws Exception{
SqlSession sqlSession = null;
try{
sqlSession = MybatisUtil.getSqlSession();
return sqlSession.selectList("studentNamespace.findAllByCourseName",name);
}catch(Exception e){
e.printStackTrace();
throw e;
}finally{
MybatisUtil.closeSqlSession();
}
}
public static void main(String[] args) throws Exception{
StudentCourseDao dao = new StudentCourseDao();
List<Course> courseList = dao.findAllByName("哈哈");
System.out.print("哈哈选学了" + courseList.size()+"个课程,分别是：");
for(Course c : courseList){
System.out.print(c.getName()+" ");
}
System.out.println("\n-----------------------------------------------------");
List<Student> studentList = dao.findAllByCourseName("android");
System.out.println("选修了android课程的学生有"+studentList.size()+"个，分别是：");
for(Student s : studentList){
System.out.print(s.getName()+" ");
}
}
}总结
对于Mybatis的多表连接就非常简单了，由于SQL语句全是由我们自己写，如果我们返回的数据类型在当前的实体中是不够封装的话，那么我们只要再关联对应的映射属性就行了！
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