# 複雜查詢環境搭建
###### tags: `Mybatis-查詢環境`

## 多對一處理
### 多對一：
* 多個學生，對應一個老師
* 對於學生而言，**關聯**，多個學生，關聯一個老師【多對一】
* 對於老師而言，**集合**，一個老師，有很多學生【一對多】

#### SQL：

CREATE TABLE `teacher` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  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');

多對一的關係，學生的TID對應老師的ID

### 測試環境搭建
1.導入lombok
2.新建實體類Teacher、Student

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

@Data
public class Student {
    private int id;
    private String name;

    //學生需要關聯一個老師！
    private Teacher teacher;
}

3.建立Mapper接口

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

4.建立Mapper.xml文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.kuang.dao.StudentMapper">



</mapper>

5.在核心配置文件中綁定註冊Mapper接口或文件

   <mappers>
       <mapper class="com.kuang.dao.TeacherMapper"/>
       <mapper class="com.kuang.dao.StudentMapper"/>
   </mappers>

6.測試查詢是否能成功

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