Version: Next

1. 多对一

多个学生对应一个老师
对于学生而言，关联多个学生，关联一个老师
对于老师而言，集合，一个老师又很多学生

之前使用resultMap中的子标签result将数据库字段映射为实体类属性

现在我们将使用resultMap的其他字段来描述更复杂的查询情况

查看官方文档对ResultMap子标签的描述：

结果映射ResultMap:

association——一个复杂类型的关联，许多结果将包装成这种类型
- 嵌套结果映射——关联本身可以是一个resultMap元素，或者从别处引用一个
collection——一个复杂类型的集合
- 嵌套结果映射——集合本身可以是一个resultMap元素，或者从别处引用一个

1.1 环境搭建

数据库表

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) VALUE (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);

1.2 新建项目

新建模块mybatis06，删除所有Mapper,Pojo，删除mybatis-config.xml中的mappers标签

1.2.1 建POJO

Teacher
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
private int id;
private String name;
}
Student
@Data
@NoArgsConstructor
@AllArgsConstructor
public class Student {
private int id;
private String name;
private Teacher teacher;
}

1.2.2 建Mapper

TeacherMapper
public interface TeacherMapper {
}
StudentMapper
public interface StudentMapper {
}

1.2.3 建mapper.xml

建立在resources目录下，与src中的Mapper接口同包（确保包是多级路径，而不是一个单级路径名字带几个.）

内容复制mybatis-config.xml，修改三个位置

把configuration和config都改为mapper即可

TeacherMapper.xml
<mapper namespace="com.bsx.dao.TeacherMapper">

</mapper>
StudentMapper.xml
<mapper namespace="com.bsx.dao.StudentMapper">

</mapper>

1.2.4 注册Mapper

在mybatis-config.xml中注册mapper

<mappers>
    <mapper resource="com/bsx/dao/TeacherMapper.xml"/>
    <mapper resource="com/bsx/dao/StudentMapper.xml"/>
</mappers>

1.2.5 测试环境搭建

接口
public interface TeacherMapper {
Teacher getTeacher(@Param("tid") int id);
}
mapper.xml
<mapper namespace="com.bsx.dao.TeacherMapper">
<select id="getTeacher" resultType="com.bsx.pojo.Teacher" >
select * from teacher where id = #{tid}
</select>
</mapper>
测试类
@Test
public void testEnv() throws IOException {
InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();

try {
TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher(1);

System.out.println(teacher);
} catch (Exception e) {
e.printStackTrace();
} finally {
sqlSession.close();
}
}

2. 多对一需求

需求，查询所有的学生信息以及对应的老师的信息

sql (sid, sname, tname, tid为别名)
SELECT
s.id sid, s.name sname, t.name tname, t.id tid
FROM
student s, teacher t
WHERE
s.tid = t.id;

Student类中有一个成员变量Teacher teacher，所以上述sql不能直接用Mybatis查询

2.1 思路一——按照查询嵌套处理

子查询

查询所有的学生信息
根据查询出来的学生的tid，寻找对应的老师

StudentMapper接口
//查询所有的学生信息以及对应的老师的信息
List<Student> getStudent();
StudentMapper.xml
主键一般用id标签，通用情况下用result标签
复杂的属性需要单独处理
- association——针对对象
  - 用javatype指定映射对象的类型
  - 用select指定嵌套查询

collection——针对集合（一对多，之后再说）

测试

@Test
public void testGetStudent() throws IOException {
    InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    SqlSession sqlSession = sqlSessionFactory.openSession();

    try {
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> students = mapper.getStudent();

        for (Student student : students) {
            System.out.println(student);
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        sqlSession.close();
    }
}

结果
Student(id=1, name=小明, teacher=Teacher(id=1, name=秦老师))
Student(id=2, name=小红, teacher=Teacher(id=1, name=秦老师))
Student(id=3, name=小张, teacher=Teacher(id=1, name=秦老师))
Student(id=4, name=小李, teacher=Teacher(id=1, name=秦老师))
Student(id=5, name=小王, teacher=Teacher(id=1, name=秦老师))

2.2 思路二——按照结果嵌套处理（常用）

查询所有的学生信息
根据查询出来的学生的tid，寻找对应的老师

接口
//方式二，按照结果嵌套
List<Student> getStudent2();
StudentMapper.xml

<select id="getStudent2" resultMap="StudentTeacher2">
SELECT
s.id sid, s.name sname, t.name tname, t.id tid
FROM
student s, teacher t
WHERE
s.tid = t.id;
</select>
<resultMap id="StudentTeacher2" type="com.bsx.pojo.Student">
<result column="sid" property="id"/>
<result column="sname" property="name"/>
<association property="teacher" javaType="Teacher">
<result column="tname" property="name"/>
<result column="tid" property="id"/>
</association>
</resultMap>
测试
@Test
public void testGetStudent2() throws IOException {
InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession sqlSession = sqlSessionFactory.openSession();

try {
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
List<Student> students = mapper.getStudent2();
for (Student student : students) {
System.out.println(student);
}
} catch (Exception e) {
e.printStackTrace();
} finally {
sqlSession.close();
}
}
结果
Student(id=1, name=小明, teacher=Teacher(id=1, name=秦老师))
Student(id=2, name=小红, teacher=Teacher(id=1, name=秦老师))
Student(id=3, name=小张, teacher=Teacher(id=1, name=秦老师))
Student(id=4, name=小李, teacher=Teacher(id=1, name=秦老师))
Student(id=5, name=小王, teacher=Teacher(id=1, name=秦老师))

1. 多对一#

1.1 环境搭建#

1.2 新建项目#

1.2.1 建POJO#

1.2.2 建Mapper#

1.2.3 建mapper.xml#

1.2.4 注册Mapper#

1.2.5 测试环境搭建#

2. 多对一需求#

2.1 思路一——按照查询嵌套处理#

2.2 思路二——按照结果嵌套处理（常用）#