Java全栈课程之Mybatis详解——多对一处理

多对一：

 多个学生，对应一个老师

对于学生而言，关联  多个学生关联一个老师 【多对一】

对于老师而言，集合  一个老师有很多学生【一对多】

SQL：

CREATE TABLE `teacher`(
`id` INT(10) NOT NULL,
`name` VARCHAR (30),
PRIMARY KEY (`id`)
)ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO teacher(`id`,`name`) VALUES (1,'秦老师');

CREATE TABLE `student`(
`id` INT (10) NOT NULL,
`name` VARCHAR (30) DEFAULT NULL,
`tid` INT (10) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `fktid` (`tid`),
CONSTRAINT fktid FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
)ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO `student`( `id`,`name`,`tid`) VALUES ('1','小明','1');
INSERT INTO `student`( `id`,`name`,`tid`) VALUES ('2','小红','1');
INSERT INTO `student`( `id`,`name`,`tid`) VALUES ('3','小张','1');
INSERT INTO `student`( `id`,`name`,`tid`) VALUES ('4','小李','1');
INSERT INTO `student`( `id`,`name`,`tid`) VALUES ('5','小王','1');

一、测试环境搭建

        1.导入Lombok

        
            org.projectlombok
            lombok
            1.18.10
        

        2.新建实体类Teacher，Student

@Data
public class Teacher {
    private int id;
    private String name;
}

@Data
public class Student {
    private int id;
    private String name;
    //学生关联一个老师
    private Teacher teacher;
}

        3.建立Mapper接口

public interface TeacherMapper {
    @Select("select * from teacher where id = #{tid}")
    Teacher geyTeacher(@Param("tid")int id);
}

public interface StudentMapper {
}

        4.建立Mapper.XML文件

        5.在核心配置文件中绑定注册接口或文件

        6.测试查询是否能成功

    public static void main(String[] args) {
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.geyTeacher(1);
        System.out.println(teacher);
        sqlSession.close();
    }

二、按照查询嵌套处理

查询所有的学生信息以及对应老师的信息

  
        select * from student
    
    
        
        
    

        
    
    
        select * from teacher where id=#{id}
    

三、按照结果嵌套处理

查询所有的学生信息以及对应老师的信息

    
        select s.id sid,s.name sname,t.name tname
        from student s,teacher t
        where s.tid = t.id