多对一处理

多对一处理

• 多个学生，对应一个老师
• 对于学生这边而言， 关联 … 多个学生，关联一个老师 【多对一】
• 对于老师而言， 集合 ， 一个老师，有很多学生 【一对多】

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');

测试环境搭建

1. 导入lombok

2. 新建实体类 Teacher，Student

   @Data
   public class Student {
   
       private int id;
       private String name;
   
       // private int tid;
       //学生关联老师
       private Teacher teacher;
   }
   

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

3. 建立Mapper接口

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

4. 建立Mapper.XML文件

   <?xml version="1.0" encoding="UTF-8" ?>
   <!DOCTYPE mapper
           PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
           "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
   
   <mapper namespace="com.saxon.dao.StudentMapper">
   
   
   </mapper>
   

5. 在核心配置文件中绑定注册我们的Mapper接口或者文件！【方式很多，随心选】

   <?xml version="1.0" encoding="UTF-8"?>
   <!DOCTYPE configuration
           PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
           "http://mybatis.org/dtd/mybatis-3-config.dtd">
   
   <configuration>
       <properties resource="db.properties"/>
   
       <settings>
           <setting name="logImpl" value="LOG4J"/>
       </settings>
   
       <typeAliases>
           <package name="com.saxon.pojo"/>
       </typeAliases>
   
       <environments default="development">
           <environment id="development">
               <transactionManager type="JDBC"/>
               <dataSource type="POOLED">
                   <property name="driver" value="${driver}"/>
                   <property name="url" value="${url}"/>
                   <property name="username" value="${username}"/>
                   <property name="password" value="${password}"/>
               </dataSource>
           </environment>
       </environments>
   
   
       <mappers>
           <mapper resource="com/saxon/dao/StudentMapper.xml"/>
           <mapper resource="com/saxon/dao/TeacherMapper.xml"/>
       </mappers>
   </configuration>
   

6. 测试查询是否能够成功！

public class TeacherMapperTest{

    @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();
    }
}

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的tid，寻找对应的老师！  子查询
    -->
<select id="getStudent" resultMap="StudentTeacher">
    select * from student
</select>
<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="getTeacher" resultType="Teacher">
    select * from teacher where id = #{id}
</select>

按照结果嵌套处理

<!--按照结果嵌套处理-->
<select id="getStudent2" resultMap="StudentTeacher2">
    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>