一、一对一

  mybatis处理一对一主要通过<resultMap>中的<association>元素来处理。

  <association>元素主要使用方方式有两种:  

<!--方式一 嵌套查询-->
<association property = "card" column = "card_id" javaType = "com.mybatis.associateMapping.IdCard"
select = "com.mybatis.mapper.IdCardMapper.findCodeById" />
<!--方式二 嵌套结果-->
<association property = "card" javaType = "com.mybatis.associateMapping.IdCard">
    <id property = "id" column = "card_id"/>
    <property = "code" column = "code"/>
</association>

  首先我们构建两张表,表结构如下:

  tb_idcard

  

  

  tb_person

  

  建表语句:

CREATE TABLE `tb_idcard` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`code` varchar(18) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
CREATE TABLE `tb_person` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(32) DEFAULT NULL,
`age` int(11) DEFAULT NULL,
`sex` varchar(8) DEFAULT NULL,
`card_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `card_id` (`card_id`),
CONSTRAINT `tb_person_ibfk_1` FOREIGN KEY (`card_id`) REFERENCES `tb_idcard` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

  tb_person表中的card_id与tb_card的id关联起来。

  现在通过id查询person信息,同时使用这个id查询对应card信息。

IdCard.java

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 + "]";
}
}

Person.java

public class Person {
private Integer id;
private String name;
private Integer age;
private String sex;
private IdCard card;
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 + "]";
} }

IdCardMapper.xml

<?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.mybatis.mapper.IdCardMapper" >
<select id = "findCodeById" parameterType = "Integer" resultType = "com.mybatis.associateMapping.IdCard">
select * from tb_idcard where id = #{id}
</select>
</mapper>

PersonMapper.xml

<?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.mybatis.mapper.PersonMapper" >
<select id = "findPersonById" parameterType = "Integer" resultMap = "IdCardWithPersonResult">
select * from tb_person where id = #{id}
</select> <resultMap type="com.mybatis.associateMapping.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 = "com.mybatis.associateMapping.IdCard"
select = "com.mybatis.mapper.IdCardMapper.findCodeById" />
</resultMap>
</mapper>
通过id查询的同时,resultMap中有一个嵌套查询会根据id查询idCard同时将查询的结果绑定到Person类的card属性上。

mybatis-config.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource = "db.properties">
</properties>
<!-- 采用扫描包定义别名 -->
<typeAliases>
<package name="com.mybatis.associateMapping"/>
</typeAliases> <!-- 配置默认环境为mysql -->
<environments default = "mysql">
<!-- 配置id为SQL的数据库环境 -->
<environment id = "my">
<!-- 设置事务管理类型为JDBC -->
<transactionManager type = "JDBC"/>
<!-- 设置数据源 -->
<dataSource type = "POOLED">
<property name = "driver" value = "${jdbc.driver}" />
<property name = "url" value = "${jdbc.url}" />
<property name = "username" value = "${jdbc.username}" />
<property name = "password" value = "${jdbc.password}" />
</dataSource>
</environment>
</environments>
<!-- 设置映射文件 -->
<mappers>
<!-- 测试mybatis -->
<mapper resource = "com/mybatis/mapper/CustomerMapper.xml"/>
<!-- mybatis关联映射 -->
<mapper resource = "com/mybatis/mapper/IdCardMapper.xml"/>
<mapper resource = "com/mybatis/mapper/PersonMapper.xml"/>
</mappers>
</configuration>

MyBatisSessionFactory.java  (工具类,用于获取SqlSessionFactory)
import java.io.IOException;
import java.io.InputStream; import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder; public class MyBatisSessionFactory {
public static SqlSessionFactory getSqlSessionFactory(String sourceFile) throws IOException {
String resource = sourceFile;
//获取配置文件输入流
InputStream inputStream = Resources.getResourceAsStream(resource);
//通过配置文件输入流构建sqlSessionFactory,
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream,"my");
return sqlSessionFactory;
}
}

测试:

import java.io.IOException;

import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory; import com.my.util.MyBatisSessionFactory; public class TestMapper {
public static void main(String[] args) throws IOException {
SqlSessionFactory sqlSessionFactory = MyBatisSessionFactory.getSqlSessionFactory("mybatis-config.xml");
SqlSession sqlSession = sqlSessionFactory.openSession();
Person person = sqlSession.selectOne("com.mybatis.mapper.PersonMapper.findPersonById",1);
System.out.println(person);
sqlSession.commit();
sqlSession.close();
}
}

通过id查询person后,通过resultMap建立返回映射时,使用id查询code同时将返回的IdCard绑定到Person中的card属性上。

上例是采用嵌套查询来实现的,下面我们使用第二种方法嵌套结果来实现。

只需修改PersonMapper.xml文件即可

<?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.mybatis.mapper.PersonMapper" >
<select id = "findPersonById" parameterType = "Integer" resultMap = "IdCardWithPersonResult">
<!-- select * from tb_person where id = #{id} -->
select p.*, idcard.code <!-- 查询语句 -->
from tb_person p,tb_idcard idcard
where p.card_id = idcard.id and p.id = #{id}
</select> <resultMap type="com.mybatis.associateMapping.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 = "com.mybatis.associateMapping.IdCard"
select = "com.mybatis.mapper.IdCardMapper.findCodeById" /> -->
<association property = "card" javaType = "IdCard">
<id property = "id" column = "id"/> <!-- 嵌套结果-->
<result property = "code" column = "code" />
</association>
</resultMap>
</mapper>

 二、一对多

   mybatis处理一对一主要通过<resultMap>中的<collection>元素来处理。

   <collection>元素主要使用方方式有两种:  

<!-- 嵌套结果-->
<collection property = "orderList" ofType = "com.mybatis.oneN.Order">
<id property = "id" column = "orders_id"/>
<result property = "number" column = "number" />
</collection>

首先创建两张表

tb_user

tb_orders

建表语句:

CREATE TABLE `tb_user` (
`id` int(32) NOT NULL AUTO_INCREMENT,
`username` varchar(32) DEFAULT NULL,
`address` varchar(256) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;
CREATE TABLE `tb_orders` (
`id` int(32) NOT NULL AUTO_INCREMENT,
`number` varchar(32) NOT NULL,
`user_id` int(32) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`),
CONSTRAINT `tb_orders_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `tb_user` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

orders中关联了用户id,即代表该商品被某一用户购买。

Order.java

public class Order {
private Integer id;
private String number;
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;
}
@Override
public String toString() {
return "Order [id=" + id + ", number=" + number + "]";
}
}

User.java

import java.util.List;

public class User {
private Integer id;
private String username;
private String address;
private List<Order> orderList; //用户购物清单 public User() {
super();
// TODO Auto-generated constructor stub
} public User(Integer id, String username, String address, List<Order> orderList) {
super();
this.id = id;
this.username = username;
this.address = address;
this.orderList = orderList;
} 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<Order> getOrderList() {
return orderList;
} public void setOrderList(List<Order> orderList) {
this.orderList = orderList;
} @Override
public String toString() {
return "User [id=" + id + ", username=" + username + ", address=" + address + ", orderList=" + orderList + "]";
} }

UserMapper.xml   (mybatis-config.xml中需导入该文件)

<?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.mybatis.mapper.UserMapper" >
<select id = "findUserWithOrders" parameterType = "Integer" resultMap = "OrdersWithUserResult">
<!-- select * from tb_person where id = #{id} -->
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="com.mybatis.oneN.User" id = "OrdersWithUserResult">
<id property="id" column="id" />
<result property="username" column="username"/>
<result property="address" column="address"/>
<collection property = "orderList" ofType = "com.mybatis.oneN.Order">
<id property = "id" column = "orders_id"/>
<result property = "number" column = "number" />
</collection>
</resultMap>
</mapper>

将嵌套结果修改为嵌套调用

UserMapper.xml

<?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.mybatis.mapper.UserMapper" >
<select id = "findUserWithOrders" parameterType = "Integer" resultMap = "OrdersWithUserResult">
select * from tb_user where id = #{id}
</select> <resultMap type="com.mybatis.oneN.User" id = "OrdersWithUserResult">
<id property="id" column="id" />
<result property="username" column="username"/>
<result property="address" column="address"/>
<collection property = "orderList" column = "id" ofType = "com.mybatis.oneN.Order"
select = "com.mybatis.mapper.OrdersMapper.findOrderByUserId"
/>
</resultMap>
</mapper>

OrdersMapper.xml

<!-- 一对多 订单表与com.mybatis.oneN.Order映射 -->
<resultMap type="com.mybatis.oneN.Order" id="OrderOneNResult" >
<id property = "id" column = "id"/>
<result property = "number" column="nuber"/>
</resultMap> <!-- 通过用户id查询订单 -->
<select id = "findOrderByUserId" parameterType = "Integer" resultMap = "OrderOneNResult">
select * from tb_orders where user_id = #{id}
</select>

三、多对多

  一个订单可以包含多个商品,同样一个商品也可以被多个订单包含。

  这种多对多关系一般需要一个中间表,用于关联两者。

  

    如上图所示,1号订单中有1,3号商品。3号商品被1,3号订单预定。两者之间通过一个中间表联系起来。

     现在我们想通过输入订单id查询该订单所包含的商品信息。例如查询一号商品订单。

    由于有一个中间表,所以我们先需要通过订单id查询中间表中Order_id为1对应的Product_id的值(1,3)

    然后通过Product_id中的(1,3)查询商品表中id为(1,3)的商品的详细信息。

  

  Product.java  (商品类,一个商品可以和多个订单关联)

import java.util.List;

public class Product {
private Integer id; //商品ID
private String name; //商品名称
private Double price; //商品价格
private List<Order> orders; //一个商品中可包含多个订单
public Product() {
// TODO Auto-generated constructor stub
} public Product(Integer id, String name, Double price, List<Order> orders) {
super();
this.id = id;
this.name = name;
this.price = price;
this.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<Order> getOrders() {
return orders;
}
public void setOrders(List<Order> orders) {
this.orders = orders;
} @Override
public String toString() {
return "Product [id=" + id + ", name=" + name + ", price=" + price + "]";
}
}

  Order.java(订单类,一个订单可以关联多个商品) 

import java.util.List;

public class Order {
private Integer id; //订单id
private String number;//订单编号
private List<Product> products; //一个订单关联多个商品 public List<Product> getProducts() {
return products;
}
public void setProducts(List<Product> products) {
this.products = products;
}
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;
}
@Override
public String toString() {
return "Order [id=" + id + ", number=" + number + ", products=" + products + "]";
}
}

  

  OrdersMapper.xml (对订单进行查询,返回订单相关信息)

<?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.mybatis.mapper.OrdersMapper"> <resultMap type="com.mybatis.MN.Order" id="OrdersWithProdecutResult">
<!-- 映射指定订单id查询到的订单id及订单编号 -->
<id property = "id" column = "id"/>
<result property = "number" column = "number" />
<!-- 嵌套查询,通过指定订单id查询关联商品 -->
<!-- column=id代表将id作为查询语句参数,查询中间表中order_id为1的product_id,并将product_id作为参数查询商品表 -->
<collection property = "products" column = "id"
ofType="com.mybatis.MN.Product"
select = "com.mybatis.mapper.ProductMapper.findProductById">
</collection>
</resultMap> <!-- 通过订单id查询该订单 -->
<select id="findOrdersWithProduct" parameterType = "Integer"
resultMap = "OrdersWithProdecutResult">
select * from tb_orders where id = #{id}
</select> </mapper>

该文件将订单id所对应的订单相关详细查询出来,并通过resultMap映射到Order类中,

Order对象中的 List<Product> products通过调用com.mybatis.mapper.ProductMapper.findProductById(ProductMapper.xml中)进行查询填充。

ProductMapper.xml

<?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.mybatis.mapper.ProductMapper">
<select id="findProductById" parameterType = "Integer"
resultType = "com.mybatis.MN.Product">
<!-- 通过商品id查询对应商品详细信息 -->
select * from tb_product where id in (
<!-- 通过订单id在中间表中查询与其关联的商品id -->
select product_id from tb_ordersitem where order_id = #{id}
)
</select> </mapper>

该文件将订单id作为查询参数,查询中间表中order_id为1所对应的product_id。(例如订单id为1,查询商品id为1,3)

然后将查询的product_id(1,3)作为参数,查询tb_orders中对应商品详细信息。

这样就通过订单id查询到了该订单对应商品的id,并将商品填充到Order类中的List<Product> products属性中。

测试:

import java.io.IOException;
import java.util.List; import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory; import com.my.util.MyBatisSessionFactory; public class TestMapper {
public static void main(String[] args) throws IOException {
SqlSessionFactory sessionFactory = MyBatisSessionFactory.getSqlSessionFactory("mybatis-config.xml");
SqlSession sqlSession = sessionFactory.openSession();
Order order = sqlSession.selectOne("com.mybatis.mapper.OrdersMapper.findOrdersWithProduct", 1);
System.out.println(order);
sqlSession.commit();
sqlSession.close();
}
}

使用嵌套结果方式:

  OrdersMapper.xml

    <resultMap type="com.mybatis.MN.Order" id="OrdersWithProdecutResult">
<!-- 映射指定订单id查询到的订单id及订单编号 -->
<id property = "id" column = "id"/>
<result property = "number" column = "number" />
<!-- 嵌套查询,通过指定订单id查询关联商品 -->
<!-- column=id代表将id作为查询语句参数 -->
<collection property = "products" ofType="com.mybatis.MN.Product">
<id property = "id" column = "pid"/>
<result property = "name" column = "name"/>
<result property = "price" column = "price"/>
</collection>
</resultMap>     <!-- 通过订单id查询该订单商品 -->
<select id="findOrdersWithProduct" parameterType = "Integer"
resultMap = "OrdersWithProdecutResult">
select o.*,p.id as pid,p.name,p.price
from tb_orders o, tb_product p, tb_ordersitem oi
where oi.order_id = o.id
and oi.product_id = p.id
and o.id = #{id}
</select>

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