springboot+mybatis一对多实现代码

假设有两张表，一张是学生表（student），一张是课程表（course），一个学生可以选多门课程，一门课程也可以被多个学生选中。这种情况下，我们可以使用一对多的关系来实现。下面是一个简单的实现代码：

1. 定义学生表（student）和课程表（course）的实体类

public class Student {
    private Long id;
    private String name;
    private List<Course> courses;
    //省略getter和setter方法
}

public class Course {
    private Long id;
    private String name;
    //省略getter和setter方法
}

2. 定义学生表（student）和课程表（course）的mapper接口

public interface StudentMapper {
    Student selectStudentById(Long id);
}

public interface CourseMapper {
    List<Course> selectCoursesByStudentId(Long studentId);
}

3. 在StudentMapper.xml中定义一对多的关系

<mapper namespace="com.example.mapper.StudentMapper">
    <resultMap id="studentMap" type="Student">
        <id column="id" property="id" />
        <result column="name" property="name" />
        <collection property="courses" ofType="Course" resultMap="courseMap" />
    </resultMap>
    
    <resultMap id="courseMap" type="Course">
        <id column="id" property="id" />
        <result column="name" property="name" />
    </resultMap>
    
    <select id="selectStudentById" resultMap="studentMap">
        SELECT s.id, s.name, c.id AS course_id, c.name AS course_name
        FROM student s
        LEFT JOIN student_course sc ON s.id = sc.student_id
        LEFT JOIN course c ON sc.course_id = c.id
        WHERE s.id = #{id}
    </select>
</mapper>

4. 在CourseMapper.xml中定义查询课程信息的SQL语句

<mapper namespace="com.example.mapper.CourseMapper">
    <select id="selectCoursesByStudentId" resultType="Course">
        SELECT c.id, c.name
        FROM course c
        LEFT JOIN student_course sc ON c.id = sc.course_id
        WHERE sc.student_id = #{studentId}
    </select>
</mapper>

5. 编写Controller和Service层代码

@RestController
public class StudentController {
    @Autowired
    private StudentService studentService;

    @GetMapping("/students/{id}")
    public Student getStudentById(@PathVariable Long id) {
        return studentService.getStudentById(id);
    }
}

@Service
public class StudentServiceImpl implements StudentService {
    @Autowired
    private StudentMapper studentMapper;
    @Autowired
    private CourseMapper courseMapper;

    @Override
    public Student getStudentById(Long id) {
        Student student = studentMapper.selectStudentById(id);
        List<Course> courses = courseMapper.selectCoursesByStudentId(id);
        student.setCourses(courses);
        return student;
    }
}

以上就是一对多的实现代码。