本文介绍了如何进行一对多查询模型，在用户和订单表关系中，展示了通过 SQL 查询用户及其订单信息，并通过 Java 代码实现了在 MyBatis 中查询所有用户信息以及用户关联的订单信息。

一对多查询模型

用户和订单表的关系为，一个用户有多个订单，一个订单只能属于一个用户。​​

一对多查询需求：查询多有用户，与此同时查询用户具有的订单信息。

​​

一对多查询语句

对应的 sql 语句

select u.*,o.ordertime,o.total,o.uid from user u left join orders o on u.id = o.uid;

查询结果如下：

id	username	password	birthday	ordertime	total	uid
1	lucy	123	2022-03-17 17:15:56	2022-03-17 17:15:33	3000	1
1	lucy	123	2022-03-17 17:15:56	2022-03-17 17:15:33	4000	1
2	tom	123	2022-03-17 17:15:56	2022-03-17 17:15:33	5000	2

代码实现

修改 User 实体

/**
 * 用户信息
 *
 * @name: User
 * @author: terwer
 * @date: 2022-03-17 17:41
 **/
public class User {
    private Integer id;
    private String username;

    // 代表当前用户具备那些订单
    private List<Order> 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 List<Order> getOrderList() {
        return orderList;
    }

    public void setOrderList(List<Order> orderList) {
        this.orderList = orderList;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username='" + username + '\'' +
                ", orderList=" + orderList +
                '}';
    }
}

创建 UserMapper

/**
 * 用户映射
 *
 * @name: UserMapper
 * @author: terwer
 * @date: 2022-03-28 00:03
 **/
public interface UserMapper {
    // 查询所有用户信息以及用户关联的订单信息
    public List<User> findAll();
}

配置 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.terwergreen.mapper.UserMapper">
    <resultMap id="userMap" type="com.terwergreen.pojo.User">
        <result property="id" column="id"></result>
        <result property="username" column="username"></result>

        <collection property="orderList" ofType="com.terwergreen.pojo.Order">
            <result property="id" column="uid"></result>
            <result property="orderTime" column="ordertime"></result>
            <result property="total" column="total"></result>
        </collection>
    </resultMap>

    <!-- resultMap：手动配置实体属性与表字段的映射关系 -->
    <select id="findAll" resultMap="userMap">
        select u.*,o.ordertime,o.total,o.uid from user u left join orders o on u.id = o.uid
    </select>
</mapper>

测试

@Test
public void test2() throws IOException {
    InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);

    SqlSession sqlSession = sqlSessionFactory.openSession();
    UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

    List<User> userList = userMapper.findAll();
    for (User user : userList) {
        System.out.println(user);
    }
}

执行结果

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文章更新历史

2024/05/13 同步文章到其他平台

2022/05/08 feat: 初稿

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