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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

   
   
            
            select * from  mybatis.student    
        
              
             
             
     
             
         
        
            select * from  mybatis.teacher where id=#{id}    
   

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

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

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);

       
            select s.id sid,s.name sname,t.name tname,t.id tid        from  mybatis.student s,mybatis.teacher t        where s.tid=t.id and t.id=#{tid}        
            
             
     
             
                 
                  
                  
              
         
    
   

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);

               select * from teacher where id=#{id}        
           
            
            
        
               select * from student where tid= #{tid}    

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：

• collection集合 一对多
• association对象 多对一

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