Mybatis 关联关系的概述(一对一，一对多，多对多)

桃扇骨 2024-04-17 19:29 13阅读 0赞

##### 1.简介 #####

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 1]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 2]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 3]

##### 2.一对一 #####

![在这里插入图片描述][20190709172033654.png]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 4]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 5]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 6]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 7]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 8]  
代码演示  
首先创建测试用的mysql数据库

use mybatis;
    create table tb_idcard(
    	id int primary key auto_increment,
        CODE varchar(18)
    );

insert into tb\_idcard(CODE)values(‘152456985247’);  
insert into tb\_idcard(CODE)values(‘152963852745’);

create table tb\_person(  
id int primary key auto\_increment,  
name varchar(32),  
age int,  
sex varchar(8),  
card\_id int unique,  
foreign key(card\_id) references tb\_idcard(id)  
);t\_usertb\_person

insert into tb\_person(name,age,sex,card\_id) values(‘Rose’,29,‘女’,1);  
insert into tb\_person(name,age,sex,card\_id) values(‘Tom’,29,‘男’,2);

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2.创建对应的实体类  
Person类

package com.lin.po;

public class Person \{

private Integer id;
 private String name;
 private Integer age;
 private String sex;
 private IdCard card;// 个人关联证件
 
 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 Integer getAge() {
 	return age;
 }
 
 public void setAge(Integer age) {
 	this.age = age;
 }
 
 public String getSex() {
 	return sex;
 }
 
 public void setSex(String sex) {
 	this.sex = sex;
 }
 
 public IdCard getCard() {
 	return card;
 }
 
 public void setCard(IdCard card) {
 	this.card = card;
 }
 
 @Override
 public String toString() {
 	return "Person [id=" + id + ", name=" + name + ", age=" + age + ", sex=" + sex + ", card=" + card + "]";
 }

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IdCard类

package com.lin.po;

public class IdCard \{

private Integer id;
 private String code;
 
 public Integer getId() {
 	return id;
 }
 
 public void setId(Integer id) {
 	this.id = id;
 }
 
 public String getCode() {
 	return code;
 }
 
 public void setCode(String code) {
 	this.code = code;
 }
 
 @Override
 public String toString() {
 	return "IdCard [id=" + id + ", code=" + code + "]";
 }

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3.创建对应的Mapper.xml文件对数据库进行操作  
首先先创建Idcard对应的xml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="com.lin.mapper.IdCardMapper">
 	
 	
 	
 	<select id="findCodeById" parameterType="Integer" resultType="IdCard">
 		select * from tb_idcard where id=#{id}
 	</select>

</mapper>

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然后创建Person的Mapper文件  
这里有两种关联查询方式  
方式一、嵌套查询:通过执行另外一条SQL映射语句来返回预期的特殊类型，比较耗内存  
编写测试类

// 嵌套查询
 	@Test
 	public void findPersonByIdTest() { 
 		// 通过工具类，生成SqlSession对象
 		SqlSession sqlSession = MybatisUtils.getSqlSession();
 	// 使用MyBatis嵌套查询的方式查询id为1的人的信息
 	Person person = sqlSession.selectOne("com.lin.mapper.PersonMapper.findPersonById", 1);
 	// 输出查询结果信息
 	System.out.println(person);
 	// 关闭SqlSession
 	sqlSession.close();
 }

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DEBUG [main] - ==> Preparing: select * from tb_person where id=? 
 DEBUG [main] - ==> Parameters: 1(Integer)
 DEBUG [main] - <== Total: 1
 DEBUG [main] - ==> Preparing: select * from tb_idcard where id=? 
 DEBUG [main] - ==> Parameters: 1(Integer)
 DEBUG [main] - <== Total: 1
 Person [id=1, name=Rose, age=29, sex=女, card=IdCard [id=1, code=152456985247]]
 
 
 
 
 
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可以看出，执行了两条SQL语句,这是我们最好在mybatis-config.xml配置文件中开启二级缓存

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这时会按需加载  
只要求得要名字  
System.out.println(person.getName());  
可以看出只执行了一条

DEBUG [main] - ==> Preparing: select * from tb_person where id=? 
 DEBUG [main] - ==> Parameters: 1(Integer)
 DEBUG [main] - <== Total: 1
 Rose

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方式二、嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集

<?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.lin.mapper.PersonMapper">
 	
 	<select id="findPersonById" parameterType="Integer" resultMap="IdCardWithPersonResult">
 		select * from tb_person where id=#{id}
 </select>
 <resultMap type="Person" id="IdCardWithPersonResult">
 	<id property="id" column="id" />
 	<result property="name" column="name" />
 	<result property="age" column="age" />
 	<result property="sex" column="sex" />
 	
 	<association property="card" column="card_id"
 		javaType="IdCard" select="com.lin.mapper.IdCardMapper.findCodeById">
 
 	</association>
 </resultMap>
 
 
 <select id="findPersonById2" parameterType="Integer"
 	resultMap="IdCardWithPersonResult">
 	select p.*,idcard.code from tb_person p,tb_idcard idcard
 	where p.card_id=idcard.id and p.id=#{id}
 </select>
 <resultMap type="Person" id="IdCardWithPersonResult2">
 	<id property="id" column="id" />
 	<result property="name" column="name" />
 	<result property="age" column="age" />
 	<result property="sex" column="sex" />
 	
 	<association property="card" javaType="IdCard">
 		<id property="id" column="card_id" />
 		<result property="code" column="code" />
 	</association>
 </resultMap>

</mapper>

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测试类

// 嵌套结果
 		@Test
 		public void findPersonByIdTest2() { 
 			// 通过工具类，生成SqlSession对象
 			SqlSession sqlSession = MybatisUtils.getSqlSession();
 		// 使用MyBatis嵌套查询的方式查询id为1的人的信息
 		Person person = sqlSession.selectOne("com.lin.mapper.PersonMapper.findPersonById2", 1);
 		// 输出查询结果信息
 		System.out.println(person);
 		// 关闭SqlSession
 		sqlSession.close();
 	}

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运行结果

DEBUG [main] - ==> Preparing: select p.*,idcard.code from tb_person p,tb_idcard idcard where p.card_id=idcard.id and p.id=? 
 DEBUG [main] - ==> Parameters: 1(Integer)
 DEBUG [main] - <== Total: 1
 Person [id=1, name=Rose, age=29, sex=女, card=IdCard [id=1, code=152456985247]]
 
 
 
 
 
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##### 3.一对多 #####

![在这里插入图片描述][20190710102744399.png]  
![在这里插入图片描述][20190710102821358.png]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 9]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 10]  
代码实例  
创建两个表

use mybatis;
    create table tb_user(
    	id int(32) primary key auto_increment,
        username varchar(32),
        address varchar(256)
    );

insert into tb\_user values(‘1’,‘詹姆斯’,‘克利夫兰’);  
insert into tb\_user values(‘2’,‘科比’,‘洛杉矶’);  
insert into tb\_user values(‘3’,‘保罗’,‘洛杉矶’);  
insert into tb\_user values(‘4’,‘库里’,‘金州’);

create table tb\_orders(  
id int(32) primary key auto\_increment,  
number varchar(32) not null,  
user\_id int(32) not null,  
foreign key(user\_id) references tb\_user(id)  
);

insert into tb\_orders values(‘1’,‘1000011’,‘1’);  
insert into tb\_orders values(‘2’,‘1000012’,‘1’);  
insert into tb\_orders values(‘3’,‘1000013’,‘2’);  
insert into tb\_orders values(‘4’,‘1000014’,‘4’);

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创建对应的实体类:  
Users类

package com.lin.po;

import java.util.List;

public class Users \{

private Integer id;
 private String username;
 private String address;
 private List<Orders> ordersList;
 
 public Integer getId() {
 	return id;
 }
 
 public void setId(Integer id) {
 	this.id = id;
 }
 
 public String getUsername() {
 	return username;
 }
 
 public void setUsername(String username) {
 	this.username = username;
 }
 
 public String getAddress() {
 	return address;
 }
 
 public void setAddress(String address) {
 	this.address = address;
 }
 
 public List<Orders> getOrdersList() {
 	return ordersList;
 }
 
 public void setOrdersList(List<Orders> ordersList) {
 	this.ordersList = ordersList;
 }
 
 @Override
 public String toString() {
 	return "User [id=" + id + ", username=" + username + ", address=" + address + ", ordersList=" + ordersList
 			+ "]";
 }

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Orders 类

package com.lin.po;

public class Orders \{

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创建UserMapper.xml文件对数据库进行操作  
一对多的查询：查看某一用户及其关联的订单信息 当关联查询出的列相同时，需要使用别用区别  
一对多关联映射：collection ofType:表示属性元素中元素的类型，List属性为Orders

<?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.lin.mapper.UserMapper">
 	
 	<select id="findUserWithOrders" parameterType="Integer" resultMap="UserWithOrdersResult">
 		select u.*,o.id as orders_id,o.number from tb_user
 		u,tb_orders o where
 		u.id=o.user_id and u.id=#{id}
 	</select>
 	<resultMap type="Users" id="UserWithOrdersResult">
 		<id property="id" column="id" />
 		<result property="username" column="username" />
 		<result property="address" column="address" />
 		
 		<collection property="ordersList" ofType="Orders">
 			<id property="id" column="orders_id" />
 			<result property="number" column="number" />
 		</collection>
 	</resultMap>
 </mapper>
 
 
 
 
 
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在mybatis-config.xml文件中添加依赖

<mappers>
 		<mapper resource="com/lin/mapper/UserMapper.xml"/>
 	</mappers>
 
 
 
 
 
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单元测试

@Test
 	public void findUserByIdTest() { 
 		// 通过工具类，生成SqlSession对象
 	SqlSession sqlSession = MybatisUtils.getSqlSession();
 
 	// 使用MyBatis嵌套查询的方式查询id为1的人的信息
 	Users user = sqlSession.selectOne("com.lin.mapper.UserMapper.findUserWithOrders", 1);
 	// 输出查询结果信息
 	System.out.println(user);
 	// 关闭SqlSession
 	sqlSession.close();
 }

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运行结果

DEBUG [main] - ==> Preparing: select u.*,o.id as orders_id,o.number from tb_user u,tb_orders o where u.id=o.user_id and u.id=? 
 DEBUG [main] - ==> Parameters: 1(Integer)
 DEBUG [main] - <== Total: 2
 User [id=1, username=詹姆斯, address=克利夫兰, ordersList=[Orders [id=1, number=1000011], Orders [id=2, number=1000012]]]

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##### 4.多对多 #####

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NDExNzI3Mg_size_16_color_FFFFFF_t_70 11]  
代码演示  
建立数据库

use mybatis;
    create table tb_product(
    	id int(32) primary key auto_increment,
        NAME varchar(32),
        price double
    );

insert into tb\_product values(‘1’,‘Java基础入门’,‘44.5’);  
insert into tb\_product values(‘2’,‘Java Web程序开发入门’,‘38.5’);  
insert into tb\_product values(‘3’,‘SSM 框架整合实战’,‘50’);

\# 创建中间表  
create table tb\_ordersitem(  
id int(32) primary key auto\_increment,  
orders\_id int(32),  
product\_id int(32),  
foreign key(orders\_id) references tb\_orders(id),  
foreign key(product\_id) references tb\_product(id)  
);  
insert into tb\_ordersitem values(‘1’,‘1’,‘1’);  
insert into tb\_ordersitem values(‘2’,‘1’,‘3’);  
insert into tb\_ordersitem values(‘3’,‘3’,‘3’);  
\#这里的tb\_orders是上面一对多的表，如果已经建立不用在建  
create table tb\_orders(  
id int(32) primary key auto\_increment,  
number varchar(32) not null,  
user\_id int(32) not null,  
foreign key(user\_id) references tb\_user(id)  
);

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建立实体类  
Product 类

package com.lin.po;

import java.util.List;

public class Product \{ 
private Integer id; 
private String name; 
private Double price; 
private List<Orders> orders;

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 Double getPrice() {
 	return price;
 }
 
 public void setPrice(Double price) {
 	this.price = price;
 }
 
 public List<Orders> getOrders() {
 	return orders;
 }
 
 public void setOrders(List<Orders> orders) {
 	this.orders = orders;
 }
 
 @Override
 public String toString() {
 	return "Product [id=" + id + ", name=" + name + ", price=" + price + ", orders=" + orders + "]";
 }

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Orders 类 多增加关联商品集合信息  
private List productList;

package com.lin.po;

import java.util.List;

public class Orders \{

private Integer id;
 private String number;
 // 关联商品集合信息
 private List<Product> productList;
 
 public Integer getId() {
 	return id;
 }
 
 public void setId(Integer id) {
 	this.id = id;
 }
 
 public String getNumber() {
 	return number;
 }
 
 public void setNumber(String number) {
 	this.number = number;
 }
 
 public List<Product> getProductList() {
 	return productList;
 }
 
 public void setProductList(List<Product> productList) {
 	this.productList = productList;
 }
 
 @Override
 public String toString() {
 	return "Orders [id=" + id + ", number=" + number + ", productList=" + productList + "]";
 }

方式一  
OrdersMapper.xml  
多对多嵌套结果查询，查询某订单及其关联的商品详情,效率高

<mapper namespace="com.lin.mapper.PersonMapper">
 	<select id="findOrderWithProduct2" parameterType="Integer" resultMap="OrdersWithProductResult2">
 		select o.*,p.id as pid,p.name,p.price from
 		tb_orders o,tb_product p,tb_ordersitem oi
 		where oi.orders_id=o.id
 		and oi.product_id=p.id
 		and o.id=#{id}
 	</select>
 	
 	<resultMap type="Orders" id="OrdersWithProductResult2">
 		<id property="id" column="id" />
 		<result property="number" column="number" />
 		
 		<collection property="productList" ofType="Product">
 			<id property="id" column="pid"/>
 			<result property="name" column="name"/>
 			<result property="price" column="price"/>
 		</collection>
 	</resultMap>
 </mapper>
 
 
 
 
 
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测试类：

/** * 多对多 嵌套结果 */
 	@Test
 	public void findOrdersByIdTest2() { 
 		// 通过工具类，生成SqlSession对象
 		SqlSession sqlSession = MybatisUtils.getSqlSession();
 	// 使用MyBatis嵌套查询的方式查询id为1的人的信息
 	Orders orders = sqlSession.selectOne("com.lin.mapper.OrdersMapper.findOrderWithProduct2", 1);
 	// 输出查询结果信息
 	System.out.println(orders);
 	// 关闭SqlSession
 	sqlSession.close();
 }

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运行结果

DEBUG [main] - ==> Preparing: select o.*,p.id as pid,p.name,p.price from tb_orders o,tb_product p,tb_ordersitem oi where oi.orders_id=o.id and oi.product_id=p.id and o.id=? 
 DEBUG [main] - ==> Parameters: 1(Integer)
 DEBUG [main] - <== Total: 2
 Orders [id=1, number=1000011, productList=[Product [id=1, name=Java基础入门, price=44.5, orders=null], Product [id=3, name=SSM 框架整合实战, price=50.0, orders=null]]]

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方式二  
OrdersMapper.xml

<mapper namespace="com.lin.mapper.PersonMapper">
 	<select id="findOrderWithProduct" parameterType="Integer" resultMap="OrdersWithProductResult">
 		select * from tb_orders where id=#{id}
 	</select>
 	<resultMap type="Orders" id="OrdersWithProductResult">
 		<id property="id" column="id" />
 		<result property="number" column="number" />
 		<collection property="productList" column="id" ofType="Product" select="com.lin.mapper.ProductMapper.findProductById">
 		</collection>
 	</resultMap>
 </mapper>
 
 
 
 
 
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ProductMapper.xml

<mapper namespace="com.lin.mapper.ProductMapper">
 	<select id="findProductById" parameterType="Integer" resultType="Product">
 		select * from tb_product where id in(
 		select product_id from tb_ordersitem where orders_id=#{id}
 		)
 	</select>
 </mapper>
 
 
 
 
 
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/** * 多对多 */
 	@Test
 	public void findOrdersByIdTest() { 
 		// 通过工具类，生成SqlSession对象
 		SqlSession sqlSession = MybatisUtils.getSqlSession();
 	// 使用MyBatis嵌套查询的方式查询id为1的人的信息
 	Orders orders = sqlSession.selectOne("com.lin.mapper.OrdersMapper.findOrderWithProduct", 1);
 	// 输出查询结果信息
 	System.out.println(orders);
 	// 关闭SqlSession
 	sqlSession.close();
 }

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运行结果：可以看出执行了两次SQL语句

DEBUG [main] - ==> Preparing: select * from tb_orders where id=? 
 DEBUG [main] - ==> Parameters: 1(Integer)
 DEBUG [main] - <== Total: 1
 DEBUG [main] - ==> Preparing: select * from tb_product where id in( select product_id from tb_ordersitem where orders_id=? ) 
 DEBUG [main] - ==> Parameters: 1(Integer)
 DEBUG [main] - <== Total: 2
 Orders [id=1, number=1000011, productList=[Product [id=1, name=Java基础入门, price=44.5, orders=null], Product [id=3, name=SSM 框架整合实战, price=50.0, orders=null]]]

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