Mybatis多对一、一对多查询

数据库表结构

实体类

StudentMapper.xml

    
        select * from student
    

    
        select * from teacher where id = #{id}
    

    
        
        
    




 
测试
 
@Test
    public void getStudent(){

        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

        List students = mapper.getStudent();

        for (Student student : students) {
            System.out.println(student);
        }

        sqlSession.close();
    }
 
 
方式二：按照结果嵌套处理 
StudentMapper.xml
 






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

    
        
        

        
            
        

    

测试

@Test
    public void getStudent02(){

        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);

        List students = mapper.getStudent02();

        for (Student student : students) {
            System.out.println(student);
        }

        sqlSession.close();
    }

TeacherMapper.xml

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

    
        

        
    

    
        select * from mybatis.student where tid = #{tid}
    

 @Test
    public void getTeacher02(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);

        Teacher teacher = mapper.getTeacher01(1);
        System.out.println(teacher);
        sqlSession.close();
    }

TeacherMapper.xml

        select s.id sid, s.name sname, t.name tname, t.id tid
        from student s ,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();
    }

1、关联 association【多对一】
2、集合 collection 【一对多】
3、javaType 和 ofType
*JavaType 用来指定实体类中属性的类型
*ofType用来指定映射到List或者集合中的实体类类型，泛型中的约束类型