Mybatis(多对一处理)

1.搭建数据库环境

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,'张老师');

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,'小沈',1);
INSERT INTO `student`(`id`,`name`,`tid`)VALUES(2,'小张',1);
INSERT INTO `student`(`id`,`name`,`tid`)VALUES(3,'小牛',1);
INSERT INTO `student`(`id`,`name`,`tid`)VALUES(4,'小白',1);
INSERT INTO `student`(`id`,`name`,`tid`)VALUES(5,'小黑',1);

2.实体类

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

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

按照查询嵌套处理

子查询

public interface StudentMapper {
    
    
    //查询所有的学生信息，以及对应的老师信息
    List<Student> getStudent();
    Teacher getTeacher(@Param("id") int id);
}

<!--
    思路：
    1.查询所有的学生信息
    2.根据查询出来的tid，寻找对应的老师，子查询
-->
<select id="getStudent" resultMap="stuGetTeacher">
   select * from mybatis.student;
</select>
    <resultMap id="stuGetTeacher" type="Student">
        <!--复杂的属性，我们需要单独处理，对象用association，集合用collection-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>

    <select id="getTeacher" resultType="Teacher">
        select *from mybatis.teacher where id=#{id};
    </select>

按照结果嵌套处理

连表查询

public interface StudentMapper {
    
    
    //查询所有的学生信息，以及对应的老师信息
    List<Student> getStudent1();
}

   <select id="getStudent1" resultMap="stuGetTeacher2">
        select  stu.id sid,stu.name sname,tea.name tname from mybatis.student stu,mybatis.teacher tea where stu.tid=tea.id;
    </select>
    <resultMap id="stuGetTeacher2" type="Student">
       <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>