Mybatis【一對多、多對1、多對多】知識要點
Mybatis【多表鏈接】
咱們在學習Hibernate的時候，若是表涉及到兩張的話，那麼咱們是在映射文件中使用<set>..<many-to-one>等標籤將其的映射屬性關聯起來的...那麼在咱們Mybatis中又怎麼作呢？？？java
先來回顧一下咱們SQL99的語法：mysql
一）內鏈接（等值鏈接）：查詢客戶姓名，訂單編號，訂單價格
---------------------------------------------------
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：內鏈接（等值鏈接）只能查詢出多張表中，鏈接字段相同的記錄
二）外鏈接：按客戶分組，查詢每一個客戶的姓名和訂單數
---------------------------------------------------
左外鏈接：
select c.name,count(o.isbn)
from customers c left outer join orders o
on c.id = o.customers_id
group by c.name;
---------------------------------------------------
右外鏈接：
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：外鏈接既能查詢出多張表中，鏈接字段相同的記錄；又能根據一方，將另外一方不符合相同記錄強行查詢出來
三）自鏈接：求出AA的老闆是EE
---------------------------------------------------
內自鏈接：
select users.ename,boss.ename
from emps users inner join emps boss
on users.mgr = boss.empno;
---------------------------------------------------
外自鏈接：
select users.ename,boss.ename
from emps users left outer join emps boss
on users.mgr = boss.empno;
---------------------------------------------------
注意：自鏈接是將一張表，經過別名的方式，看做多張表後，再進行鏈接。
這時的鏈接便可以採用內鏈接，又能夠採用外鏈接
因爲咱們Mybatis中並無像Hibernate這樣全自動化的，所以咱們是沒有<set>..<many-to-one>等標籤的，咱們仍是使用手寫SQL語句來使咱們的關聯屬性鏈接起來...android
一對一
需求：sql
- 學生和身份證
設計表：
--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;
實體
/**
* 身份證(單方)
* @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;
}
}
/**
* 學生(單方)
* @author AdminTC
*/
public class Student {
private Integer id;
private String name;
private Card card;//關聯屬性
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
因爲咱們有兩個實體，所以咱們會有兩個映射文件數據庫
Student映射文件apache
<?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映射文件微信
<?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層
如今我想根據學生的編號查詢學生的信息和身份證信息！session
因爲該查詢着重是查詢學生的信息，因而咱們在學生的映射文件中寫SQL語句mybatis
按照需求，咱們寫出來的SQL語句是這樣子的。app
select * from zhongfucheng.students s,zhongfucheng.cards c where c.cid = s.scid and sid=1;
我來看一下查詢結果：
咱們的實體與映射表中，Student實體是沒有關聯其餘的字段的，僅僅是寫出了該實體的自帶的屬性。
<resultMap type="zhongfucheng2.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
</resultMap>
明顯地，咱們Student是不能封裝返回的結果，所以咱們須要將關聯屬性進行關聯起來！
<resultMap type="zhongfucheng2.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
<!--
property寫的是在Student實體中寫關聯字段的屬性變量名稱
resultMap寫的是映射文件中的命名空間.id
-->
<association property="card" resultMap="cardNamespace.cardMap"/>
</resultMap>
咱們關聯了之後，Student實體就可以封裝返回的結果了
<resultMap type="zhongfucheng2.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
<!--
property寫的是在Student實體中寫關聯字段的屬性變量名稱
resultMap寫的是映射文件中的命名空間.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>
查詢編號爲1的學生信息【包括身份證編號】
public Student findById(int id) throws Exception {
//獲得鏈接對象
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(1);
System.out.println(student.getId() + "----" + student.getName() + "----" + student.getCard().getNum());
}
一對多
需求：
- 一個班級有多個學生,查詢java學科有哪些學生信息
設計數據庫表
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;
實體
package zhongfucheng2;
import java.util.ArrayList;
import java.util.List;
/**
* 學科(單方)
* @author AdminTC
*/
public class Grade {
private Integer id;
private String name;
private List<Student> studentList = new ArrayList<Student>();//關聯屬性
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;
/**
* 學生(多方)
* @author AdminTC
*/
public class Student {
private Integer id;
private String name;
private Grade grade;//關聯屬性
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語句
<mapper namespace="studentNamespace">
<resultMap type="zhongfucheng2.Student" id="studentMap">
<id property="id" column="sid"/>
<result property="name" column="sname"/>
</resultMap>
<!--查詢選修的java學科有多少位學生-->
<!--因爲咱們只要查詢學生的名字，而咱們的實體studentMap能夠封裝學生的名字，那麼咱們返回studentMap便可，並不須要再關聯到學科表-->
<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 {
//獲得鏈接對象
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());
}
}
多對多
需求：
- 學生和課程
數據庫表
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;
實體
package cn.itcast.javaee.mybatis.many2many;
import java.util.ArrayList;
import java.util.List;
/**
* 課程(多方)
* @author AdminTC
*/
public class Course {
private Integer id;
private String name;
private List<Student> studentList = new ArrayList<Student>();//關聯屬性
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;
/**
* 學生(多方)
* @author AdminTC
*/
public class Student {
private Integer id;
private String name;
private List<Course> courseList = new ArrayList<Course>();//關聯屬性
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>
<!-- 查詢哈哈選學了哪些課程 -->
<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;
/**
* 持久層
* @author AdminTC
*/
public class StudentCourseDao {
/**
* 查詢哈哈選學了哪些課程
* @param name 表示學生的姓名
*/
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();
}
}
/**
* 查詢java課程有哪些學生選修
* @param name 表示學生的課程
*/
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("哈哈選學了" + courseList.size()+"個課程,分別是：");
for(Course c : courseList){
System.out.print(c.getName()+" ");
}
System.out.println("\n-----------------------------------------------------");
List<Student> studentList = dao.findAllByCourseName("android");
System.out.println("選修了android課程的學生有"+studentList.size()+"個，分別是：");
for(Student s : studentList){
System.out.print(s.getName()+" ");
}
}
}
總結
對於Mybatis的多表鏈接就很是簡單了，因爲SQL語句全是由咱們本身寫，若是咱們返回的數據類型在當前的實體中是不夠封裝的話，那麼咱們只要再關聯對應的映射屬性就好了！
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