Mybatis_06 对应关系

多对一： 使用关联 association

一对多： 使用集合 collection

创建SQL表:

CREATE TABLE `teacher` (
`id` INT(10) NOT NULL,
`name` VARCHAR(30) DEFAULT NULL,
PRIMARY KEY (`id`)
)ENGINE=INNODB DEFAULT CHARSET=utf8

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','小赵','1');
INSERT INTO `student`(`id`,`name`,`tid`) VALUES('2','小李','1');
INSERT INTO `student`(`id`,`name`,`tid`) VALUES('3','小张','1');
INSERT INTO `student`(`id`,`name`,`tid`) VALUES('4','小红','1');
INSERT INTO `student`(`id`,`name`,`tid`) VALUES('5','小明','1');

INSERT INTO `teacher`(`id`,`name`) VALUES('1','赵老师');

表的关系：

测试环境搭建：

1、新建实体类Teacher、Student

2、建立Mapper接口，StudentMapper,TeacherMapper

3、建立Mapper.xml文件

4、在核心配置文件中绑定Mapper接口

5、测试查询是否成功

mapper接口

mapper对应的mapper.xml文件

多对一：

按照查询嵌套处理：

<!--
  思路：
      1、查询所有的学生信息
      2、根据查出来的学生的tid，查询对应的老师！
  -->
<select id="getStudent" resultMap="StudentTeacher">
    select * from mybatis.student;
</select>
<resultMap id="StudentTeacher" type="Student">
    <result property="id" column="id"/>
    <result property="name" column="name"/>
    <!--   复杂的属性需要单独处理
           对象: association
           集合: collection
         -->
    <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
</resultMap>

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

按照结果嵌套处理：

<select id="getStudent2" resultMap="StudentTeacher2">
    select s.id sid,s.name sname,t.name tname
    from mybatis.student s,mybatis.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>

Mysql 多对一查询方式：

• 子查询
• 联表查询

一对多：

新建实体类：

Student：

public class Student {
    private int id;
    private String name;
    private int tid;

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", tid=" + tid +
                '}';
    }
}

按照结果嵌套处理：

TeacherMapper:

public interface TeacherMapper {

//    List<Teacher> getTeacher();

    //获取指定老师下的所有学生
    Teacher getTeacher(@Param("tid") int id);

    Teacher getTeacher2(@Param("tid") int id);
}

<!-- 按结果嵌套查询 -->
<select id="getTeacher" resultMap="TeacherStudent">
    select s.id sid,s.name sname,t.name tname,t.id tid
    from mybatis.student s,mybatis.teacher t
    where s.tid=t.id and t.id=#{tid};
</select>
<resultMap id="TeacherStudent" type="Teacher">
    <result property="id" column="tid"/>
    <result property="name" column="tname"/>
    <collection property="students" ofType="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <result property="tid" column="tid"/>
    </collection>
</resultMap>

按照查询嵌套处理：

<!--子查询-->
<select id="getTeacher2" resultMap="TeacherStudent2">
    select * from mybatis.teacher where id= #{tid};
</select>
<resultMap id="TeacherStudent2" type="Teacher">
    <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherID" column="id"/>
</resultMap>

<select id="getStudentByTeacherID" resultType="Student">
    select *
    from mybatis.student
    where tid=#{tid};
</select>

测试：

@Test
public void test2(){
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
    Teacher teacher = mapper.getTeacher(1);
    System.out.println(teacher);
    sqlSession.close();
}
@Test
public void test3(){
    SqlSession sqlSession = MybatisUtils.getSqlSession();
    TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
    Teacher teacher = mapper.getTeacher2(1);
    System.out.println(teacher);
    sqlSession.close();
}

总结：

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