#MyBatis_一以多查询 @FDDLC
一个用户可能对应多个账户，因此用户与账户存在一对多的关系。
关键配置：
<resultMap id="userWithAccount" type="cn.liuxingchang.domain.User">
<id property="id" column="id" />
<result property="name" column="name" />
<result property="age" column="age" />
<collection property="accounts" ofType="cn.liuxingchang.domain.Account">
<id property="id" column="aid" />
<result property="money" column="money" />
</collection>
</resultMap>
<select id="findAll" resultMap="userWithAccount">
select u.*, a.id as aid, money from user u left outer join account a on u.id = a.uid
</select>
说明：
1、上面这个findAll的结果是List集合(List<User>)。
2、要用左外连接left outer join(如果user表在account表右边就用右外连接)，不能用where接条件，而是用on！
3、一个用户可能有多个账户，因此User类里有个类型为List<Account>的accounts成员。给User类中的id、name、age赋值很简单，怎么把数据封装到这个accounts成员呢？于是来了个配了个resultMap标签。resultMap标签里的collection子标签是重头戏！MyBatis就是通过collection来给accounts成员赋值的！collection标签里的property="accounts"好理解，就是User类里的accounts成员呗！ofType属性是说集合里的元素属于什么类型。另外，这个collection其实相当于是一种嵌套版的resultMap标签哈！
最后附上源码：
项目结构：
数据库的相关信息：
MySQL：8.0.16 port：3306 database：multi_table
account表：
user表：
Account类：
package cn.liuxingchang.domain;
import java.io.Serializable;
public class Account implements Serializable {
private Integer id;
private Integer uid;
private Double money;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public Integer getUid() {
return uid;
}
public void setUid(Integer uid) {
this.uid = uid;
}
public Double getMoney() {
return money;
}
public void setMoney(Double money) {
this.money = money;
}
@Override
public String toString() {
return "Account{" +
"id=" + id +
", uid=" + uid +
", money=" + money +
'}';
}
}
User类：
package cn.liuxingchang.domain;
import java.io.Serializable;
import java.util.List;
public class User implements Serializable {
private Integer id;
private String name;
private Integer age;
private List<Account> accounts;
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 List<Account> getAccounts() {
return accounts;
}
public void setAccounts(List<Account> accounts) {
this.accounts = accounts;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", age=" + age +
", accounts=" + accounts +
'}';
}
}
UserDao接口：
package cn.liuxingchang.dao;
import cn.liuxingchang.domain.User;
import java.util.List;
public interface UserDao {
List<User> findAll();
}
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="cn.liuxingchang.dao.UserDao">
<resultMap id="userWithAccount" type="cn.liuxingchang.domain.User">
<id property="id" column="id" />
<result property="name" column="name" />
<result property="age" column="age" />
<collection property="accounts" ofType="cn.liuxingchang.domain.Account">
<id property="id" column="aid" />
<result property="money" column="money" />
</collection>
</resultMap>
<select id="findAll" resultMap="userWithAccount">
select u.*, a.id as aid, money from user u left outer join account a on u.id = a.uid
</select>
</mapper>
mybatis.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>
<environments default="mysql">
<environment id="mysql">
<transactionManager type="JDBC"></transactionManager>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.cj.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/multi_table?characterEncoding=UTF8&serverTimezone=Asia/Shanghai"/>
<property name="username" value="root"/>
<property name="password" value="root"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="mapper/UserMapper.xml" />
</mappers>
</configuration>
pom.xml：
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>org.example</groupId>
<artifactId>P075_MyBatis_One2Many</artifactId>
<version>1.0-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.16</version>
</dependency>
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.4.5</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
</dependencies>
</project>
Test.java：
import cn.liuxingchang.dao.UserDao;
import cn.liuxingchang.domain.User;
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 org.junit.After;
import org.junit.Before;
import java.io.InputStream;
import java.util.List;
public class Test {
private InputStream in;
private SqlSessionFactory factory;
private SqlSession sqlSession;
private UserDao userDao;
@Before
public void before() throws Exception {
in = Resources.getResourceAsStream("mybatis.xml");
factory = new SqlSessionFactoryBuilder().build(in);
sqlSession = factory.openSession();
userDao = sqlSession.getMapper(UserDao.class);
}
@After
public void after() throws Exception {
sqlSession.commit();
sqlSession.close();
in.close();
}
@org.junit.Test
public void findAllTest() {
List<User> users = userDao.findAll();
for (User user : users) {
System.out.println(user);
}
}
}