Mybatis之一对多和多对一查询

Mybatis之一对多和多对一查询

什么是一对多？什么是多对一？

创建表：

-- 用户表
CREATE TABLE `user1`(
`id` INT PRIMARY KEY AUTO_INCREMENT,
`user_name` VARCHAR(20),-- 用户姓名
`address` VARCHAR(60)-- 联系地址
) ENGINE INNODB CHARSET utf8;
INSERT INTO `user1` VALUES(1,'陈大','深圳市南山区');
INSERT INTO `user1` VALUES(2,'王二','深圳市福田区');
INSERT INTO `user1` VALUES(3,'张三','深圳市龙华新区');
INSERT INTO `user1` VALUES(4,'李四','深圳市龙岗区');

-- 卡表
CREATE TABLE `card1`(
`id` INT PRIMARY KEY AUTO_INCREMENT,
`card_no` VARCHAR(18),
`remark` VARCHAR(100),
`user_id` INT-- 用于关联user1的主键id(不设置外键，避免级联问题)
) ENGINE=INNODB CHARSET=utf8;
INSERT INTO `card1` VALUES(1,'420001','工资卡',1);
INSERT INTO `card1` VALUES(2,'420002','工资卡',2);
INSERT INTO `card1` VALUES(3,'420003','工资卡',3);
INSERT INTO `card1` VALUES(4,'420004','工资卡',3);

-- SELECT * FROM `user1`;
-- SELECT * FROM `card1`;

实体类:Card1

public class Card1 {
private int id;
private String cardNo;
private String remark;

//...省略setter、getter方法
}

实体类：User1

public class User1 {
private int id;
private String userName;
private String address;
private List<Card1> cards;

public String toString(){
return "[ id = " + id  + ", userName = "
+ userName + ", address = " + address + "]";
}
//...省略setter、getter方法
}

实体类User1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="com.chensan.o2m.entity.User1Mapper">
<resultMap id="user1" type="com.chensan.o2m.entity.User1">
<id property="id" column="user_id"/>
<result property="userName" column="user_name"/>
<result property="address" column="address"/>

<collection property="cards" column="user_id" ofType="com.chensan.o2m.entity.Card1">
<id property="id" column="id"/>
<result property="cardNo" column="card_no"/>
<result property="remark" column="remark"/>
</collection>
</resultMap>

<select id="query" parameterType="int" resultMap="user1">
SELECT t1.`id` `user_id`,t1.`user_name`,t1.`address`,t2.`id`,t2.`card_no`,t2.`remark`
FROM `user1` t1,`card1` t2
WHERE t1.`id`=t2.`user_id` AND t1.`id`=#{id}
</select>
</mapper>

myBatis配置文件

<?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>
<!-- 和spring整合后 environments配置将废除-->
<environments default="development">
<environment id="development">
<!-- 使用jdbc事务管理  或者JTA事务管理-->
<transactionManager type="JDBC" />
<!-- 数据库连接池  第三方组件：c3p0-->
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/mybatis01"/>
<property name="username" value="root"/>
<property name="password" value="123456"/>
</dataSource>
</environment>
</environments>

<!-- 加载实体类的映射文件 -->
<mappers>
<mapper resource="com/chensan/o2m/mapper/User1Mapper.xml"/>
</mappers>
</configuration>

测试类

import java.io.Reader;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import com.chensan.o2m.entity.Card1;
import com.chensan.o2m.entity.User1;

public class TestO2M {
private static SqlSessionFactory sqlSessionFactory;
private static Reader resource;

//创建会话工厂
static{
try{
resource = Resources.getResourceAsReader("myBatisConfig.xml");
sqlSessionFactory = new SqlSessionFactoryBuilder().build(resource);
}catch(Exception e){
e.printStackTrace();
}
}

public static SqlSessionFactory getSession(){
return sqlSessionFactory;
}

//一对多：查询用户对应卡（银行卡）
public void getUserCard(){
SqlSession sqlSession = sqlSessionFactory.openSession();
User1 user = sqlSession.selectOne("com.chensan.o2m.entity.User1Mapper.query", 3);
System.out.println(user);
for(Card1 card : user.getCards()){
System.out.println(
"[ " +
"userId = " + user.getId() + ", " +
"userName = " + user.getUserName() + ", " +
"address = " + user.getAddress() + ", " +
"cardId = " + card.getId() + ", " +
"cardNo = " + card.getCardNo() + ", " +
"remark = " + card.getRemark() +
" ]"
);
}

sqlSession.close();
}

public static void main(String[] args) {
TestO2M testMyBatisOneToMany = new TestO2M();
testMyBatisOneToMany.getUserCard();
}
}

结果

[ id = 3, userName = 张三, address = 深圳市龙华新区]
[ userId = 3, userName = 张三, address = 深圳市龙华新区, cardId = 3, cardNo = 420003, remark = 工资卡 ]
[ userId = 3, userName = 张三, address = 深圳市龙华新区, cardId = 4, cardNo = 420004, remark = 工资卡 ]

user1和card1的id列同名，如果不对其中一列取别名，则user只关联一个card对象，只有一条记录：

[ id = 3, userName = 张三, address = 深圳市龙华新区]
[ userId = 3, userName = 张三, address = 深圳市龙华新区, cardId = 3, cardNo = 420003, remark = 工资卡 ]