哪吒Java技能树--MyBaits学习笔记--关联和集合对应着多对一和一对多（“最易懂得MyBatis学习”）（上）

#数据库里面的`teacher`是键盘Tab上面的，而'王老师'这里的'是回车左边的
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, '王老师'); 
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);

<dependencies>
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>1.18.20</version>
    </dependency>
</dependencies>

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

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

import com.hxl.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;
public interface TeacherMapper {
    @Select("select * from teacher where id = #{tid}")
    Teacher getTeacher(@Param("tid")int id);
}

public interface StudentMapper {
}

<?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">
<!--核心配置文件-->
<mapper namespace="com.hxl.dao.TeacherMapper">
</mapper>

<?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">
<!--核心配置文件-->
<mapper namespace="com.hxl.dao.StudentMapper">
</mapper>

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

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

public interface StudentMapper {
    //查询所有的学生信息，以及对应的老师的信息
    public List<Student> getStudent();
}

<mapper namespace="com.hxl.dao.StudentMapper">
    <!--方式一：按照查询嵌套处理
    思路：
        1.查询所有的学生信息
        2.根据查询出来的学生tid，寻找对应的老师
    -->
    <resultMap id="studentTeacher" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--复杂的属性，我们需要单独处理. 对象：association   集合：collection -->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
    <select id="getStudent" resultMap="studentTeacher">
        select * from student;
    </select>
    <select id="getTeacher" resultType="Teacher">
        select * from teacher where id = #{id};
    </select>
</mapper>

<!--方式二：按照结果嵌套处理-->
<select id="getStudent2" resultMap="Student2">
    select s.id sid, s.name sname, t.name tname from student s, teacher t where s.tid = t.id;
</select>
<resultMap id="StudentTeacher2" type="Student">
    <result property="id" column="sid"/>
    <result property="name" column="sname"/>
    <association property="teacher" javaType="Teacher">
        <result property="name" column="tname"/>
    </association>
</resultMap>