MyBatis——实现复杂查询

多对一情况下实现复杂查询

对学生而言：多个学生关联一个老师

1、环境搭建

1.1、建表插数据

CREATE TABLE `teacher` (  `id` INT(10) NOT NULL,  `name` VARCHAR(30) DEFAULT NULL,  PRIMARY KEY (`id`)) ENGINE=INNODB DEFAULT CHARSET=utf8INSERT INTO teacher(`id`, `name`) VALUES (1, '秦老师'); INSERT INTO teacher(`id`, `name`) VALUES (2, '沈老师'); 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=utf8INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('1', '小明', '1'); INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('2', '小红', '2'); INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('3', '小张', '1'); INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('4', '小李', '2'); INSERT INTO `student` (`id`, `name`, `tid`) VALUES ('5', '小王', '1');

1.2、导入Lombok依赖，编写实体类


               
    
     org.projectlombok
                
    
     lombok
                
    
     1.18.16

Teacher

@Datapublic class Teacher {       private int id;    private String name;}

Student

@Datapublic class Student {       private int id;    private String name;    //每个学生都需要关联一个老师，所以在每个学生里面都有一个老师对象    private Teacher teacher;}

1.3、编写接口和Mapper.xml文件

//StudentMapperpublic interface StudentMapper {       //查询所有学生的信息和老师的名字     List
   
     getStudent();}

//TeacherMapperpublic interface TeacherMapper {       Teacher gerTeacher( int id);}

2、用子查询实现

测试

@Test    public void getStudent(){           SqlSession sqlSession = MybatisUtils.getSqlSession();        StudentMapper mapper =  sqlSession.getMapper(StudentMapper.class);        List
   
     studentList = mapper.getStudent();        for (Student student : studentList) {               System.out.println(student);        }        sqlSession.close();    }

3、用连表查询实现

测试

@Test    public void getStudent2(){           SqlSession sqlSession = MybatisUtils.getSqlSession();        StudentMapper mapper =  sqlSession.getMapper(StudentMapper.class);        List
   
     studentList = mapper.getStudent2();        for (Student student : studentList) {               System.out.println(student);        }        sqlSession.close();    }

一对多情况下实现复杂查询

对老师而言：一个老师拥有多个学生

1、环境搭建

环境搭建除了实体类不一样，其他和一对多一样

Teacher

@Datapublic class Teacher {       private int id;    private String name;        //一个老师拥有多个学生    private List
   
     students;}

Student

@Datapublic class Student {       private int id;    private String name;    private int tid;}

2、用连表查询实现

//获取指定老师下的所有学生及老师的信息    Teacher getTeacher(@Param("tid") int id);

测试

@Test    public void getTeacher(){              SqlSession sqlSession = MybatisUtils.getSqlSession();           TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);           Teacher teacher = mapper.getTeacher(1);           System.out.println(teacher);           sqlSession.close();       }

2、用子查询实现

//获取指定老师下的所有学生及老师的信息    Teacher getTeacher2(@Param("tid") int id);

测试

@Test    public void getTeacher2(){           SqlSession sqlSession = MybatisUtils.getSqlSession();        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);        Teacher teacher = mapper.getTeacher(1);        System.out.println(teacher);        sqlSession.close();    }

总结：