Mybatis 一对多关联查询

方法一、多表查询（一次性查询）

跟一对一配置差不多

一个用户拥有多种角色

创建数据脚本

# 用户表
create table user(
	id int(11) primary key auto_increment,
	name varchar(20),
	age int(2)
)engine=InnoDB default charset=utf8;

# 角色表
create table role(
	id int(11) primary key auto_increment,
	name varchar(20),
	user_id int(11)
)engine=InnoDB default charset=utf8;

创建实体类

@Data
public class User implements Serializable {
    
    

    private Integer id;

    private String name;

    private Integer age;

    private IdCard idCard;

    // 一个用户拥有多种角色
    private List<Role> roleList;
}

@Data
public class Role implements Serializable {
    
    

    private Integer id;

    private String name;
}

映射文件配置

<?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.test.quickstart.mapper.UserMapper">

    <!-- 开启二级缓存 -->
    <cache />

    <!-- 结果集映射 -->
    <resultMap id="UserMap" type="User">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="age" property="age" />
    </resultMap>

    <!-- 创建sql片段，通过include引用-->
    <sql id="BaseColumnList">
        id, name, age
    </sql>

    <resultMap id="UserAndRoleMap" type="User" extends="UserMap">
        <!--
             collection：关联映射
             property: 映射的属性值
             javaType: 映射的类型
             ofType: 映射实体bean类型,这里使用类型别名
             columnPrefix: 指定关联表列前缀。好处:避免列同名出现数据出错
          -->
        <collection property="roleList" javaType="java.util.ArrayList" ofType="Role" columnPrefix="role_">
            <id column="id" property="id"/>
            <result column="name" property="name"/>
        </collection>
    </resultMap>

    <select id="selectOne2Many" resultMap="UserAndRoleMap">
         select
          u.id,
          u.name,
          u.age,
          r.id as role_id,
          r.name as role_name
          from user as u,role as r
          where u.id=r.user_id
    </select>
</mapper>

配置文件参考 mybatis-config.xml

方法二、使用关联的XXMapper对象方法嵌套查询（多次查询）

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.test.quickstart.mapper.UserMapper">

    <!-- 开启二级缓存 -->
    <cache />

    <!-- 结果集映射 -->
    <resultMap id="UserMap" type="User">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
        <result column="age" property="age" />
    </resultMap>

    <!-- 创建sql片段，通过include引用-->
    <sql id="BaseColumnList">
        id, name, age
    </sql>

    <resultMap id="UserAndRoleMap" type="User" extends="UserMap">
        <!--
             collection：关联映射
             property: 映射的属性值
             javaType: 映射的类型
             ofType: 映射实体bean类型,这里使用类型别名
             column: 指定主查询中列的结果作为嵌套查询的参数
             多参数时在column中用"{}"将参数包起来,=左侧的为mapper中定义的@Param,=右侧为主表列名
             fetchType: lazy 延迟加载,用到该对象时才发送sql查询
             默认值是eager,将发出sql,查找关联数据
          -->
        <collection property="roleList" fetchType="lazy" javaType="java.util.ArrayList" ofType="Role"
            column="{userId=id}" select="com.test.quickstart.mapper.RoleMapper.selectByUserId"/>
    </resultMap>

    <select id="selectOne2Many2" resultMap="UserAndRoleMap">
         select * from user;
    </select>
</mapper>

RoleMapper.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.test.quickstart.mapper.RoleMapper">

    <!-- 结果集映射 -->
    <resultMap id="RoleMap" type="Role">
        <id column="id" property="id"/>
        <result column="name" property="name"/>
    </resultMap>

    <select id="selectByUserId" resultMap="RoleMap">
         select * from role where user_id = #{userId}
    </select>
</mapper>

UserMapper.java

@Mapper
public interface UserMapper {
    
    
    User selectOne2Many(@Param("id") Integer id);

    User selectOne2Many2(@Param("id") Integer id);
}

RoleMapper.java

@Mapper
public interface RoleMapper {
    
    

    List<Role> selectByUserId(@Param("userId") Integer userId);
}

测试

public class Demo01Test {
    
    

    private SqlSessionFactory sessionFactory;

    @Before
    public void before() throws IOException {
    
    
        // 读取配置文件
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        sessionFactory = new SqlSessionFactoryBuilder().build(is);
    }

    /**
     * 测试一对多
     */
    @Test
    public void testOne2Many1() {
    
    
        SqlSession session = sessionFactory.openSession();
        UserMapper userMapper = session.getMapper(UserMapper.class);
        User user = userMapper.selectOne2Many(2);
        System.out.println(user);
    }
    
    /**
     * 测试一对多
     */
    @Test
    public void testOne2Many2() {
    
    
        SqlSession session = sessionFactory.openSession();
        UserMapper userMapper = session.getMapper(UserMapper.class);
        User user = userMapper.selectOne2Many2(2);
        // 使用关联对象时才发出sql查询
        System.out.println(user.getRoleList());
    }
}