介绍

collection是封装一对多关系的，通常情况下是一个列表，association是一对一，通常情况是一个对象。例如：查询班级下所有的学生，一个班级可以有多个学生，这就是一对多。

案例

有一个学生表，一个班级表，需求根据班级表查出该班级下所有的学生

一对多查询

SELECT
s.name studentsName ,
s.classes,
s.id studentsId,
c.id ,
c.name FROM classes c 
JOIN students s ON c.id=2 AND c.id=s.classes

实体类

学生实体

@Data
public class Students {Long id;String name;Long classes;//班级消息Classes belong;
}

班级实体

@Data
public class Classes {Long id;String name;List<Students> students;
}

controller

@RestController
@RequestMapping("/classes")
public class ClassesController {@AutowiredClassesMapper classesMapper;@GetMapping("/getClass")public Result getClass(Integer id){return  Result.success("获取成功",classesMapper.list(id););}}

Mapper

@Mapper
public interface ClassesMapper {Classes list(Integer id);
}

<select id="list" resultMap="studentList" >SELECT   s.name studentsName ,s.classes,s.id studentsId,c.id ,c.name FROM classes c JOIN students s ON c.id=#{id} AND c.id=s.classes
</select><resultMap id="studentList" type="com.example.demo.demos.web.pojo.Classes" ><id column="id" property="id"></id><result column="name" property="name"></result><collection property="students" ofType="com.example.demo.demos.web.pojo.Students" ><id property="id" column="studentsId"></id><result property="name" column="studentsName"></result>            <result property="classes" column="classes" ></result></collection></resultMap>

查询结果

{"code": 200,"msg": "获取成功","data": {"id": 3,"name": "大数据2","students": [{"id": 2,"name": "赵子龙","classes": 3,"belong": null},{"id": 4,"name": "马冬梅","classes": 3,"belong": null},{"id": 5,"name": "易烊干玺","classes": 3,"belong": null}]}
}