1.一对多：

场景设置：
一个计划(Plan)下有多个任务(Task)，即一对多的关系

1.1 TaskEntity.java

import lombok.Data;
@Data
public class TaskEntity {
    private int id;
    private int planId;
    private String missionId;
}

1.2 PlanEntity.java

import lombok.Data; 
import java.util.List;

@Data
public class PlanEntity {
    private int id;
    private String planName;
    private List<TaskEntity> tasks;
}

1.3 PlanMapper.java

@Repository
public interface PlanMapper extends MybatisSimpleBaseMapper<PlanEntity,Integer> {
    PlanEntity selectById(@Param("id") Integer id);
}

1.4 TaskMapper.java

import java.util.List;
public interface TaskMapper extends MybatisSimpleBaseMapper<TaskEntity, Integer> {
    List<TaskEntity> selectByPlanId(@Param("planId") int planId);
}

1.5 TaskMapper.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="TaskMapper">
    <resultMap id="BaseResultMap"
               type="TaskEntity">
        <id column="id" jdbcType="INT" property="id"/>
        <result column="plan_id" jdbcType="VARCHAR" property="planId"/>
        <result column="mission_id" jdbcType="VARCHAR" property="missionId"/>
    </resultMap>

    <select id="selectByPlanId" resultMap="BaseResultMap">
        select *
        from task
        where plan_id = #{planId}
    </select>

</mapper>

1.6 PlanMapper.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="PlanMapper">
    <resultMap id="BaseResultMap" type="PlanEntity">
        <id column="id" jdbcType="INTEGER" property="id"/>
        <result column="plan_name" jdbcType="VARCHAR" property="planName"/>
        <collection property="tasks" select="TaskMapper.selectByPlanId" column="id"/>
    </resultMap>

    <select id="selectById" resultMap="BaseResultMap">
        select *
        from plan
        where id = #{id}
    </select>

</mapper>

1.7 单元测试类：PlanMapperTest.java

@Setter
@Slf4j
@RunWith(SpringJUnit4ClassRunner.class)
@SpringBootTest(classes = ApplicationLoader.class)
@WebAppConfiguration
public class PlanMapperTest {

    @Autowired
    private PlanMapper planMapper;

    @Test
    public void selectById() {
       PlanEntity planEntity = planMapper.selectById(168);
       log.info("planEntity:{}", JSON.toJSONString(planEntity));
    }
 
}