多对一处理

生成数据

以学生老师为例

CREATE TABLE `teacher` (
    `id` INT(11) NOT NULL,
    `name` VARCHAR(30) DEFAULT NULL,
    PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8
INSERT INTO teacher(`id`, `name`) VALUES (1, '李永乐');
CREATE TABLE `student` (
    `id` INT(11) NOT NULL,
    `name` VARCHAR(30) DEFAULT NULL,
    `tid` INT(11) NOT 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),(2, '小红', 1),(3, '小张', 1),(4, '小李', 1),(5, '小王', 1)

测试环境搭建

导入lombok

<dependency>
 <groupId>org.projectlombok</groupId>
 <artifactId>lombok</artifactId>
 <version>1.16.18</version>
 <scope>provided</scope>
</dependency>

新建实体类Teacher，Student

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

@Data
public class Student {
    private Integer id;
    private String name;
    private Teacher teacher;
}

建立Mapper接口

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

public interface StudentMapper {
}

建立Mapper.xml文件 ```xml <?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">


```xml
<?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">
<mapper namespace="com.tassel.dao.StudentMapper">

</mapper>

在核心配置文件中绑定注册我们的Mapper接口或者文件

<mappers>
 <!-- 每一个mapper配置文件都要在mybatis核心配置文件中注册 -->
 <mapper class="com.tassel.dao.TeacherMapper"/>
</mappers>

测试

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

环境搭建成功。

多对一实例

案例：查询所有的学生和对应的老师信息，多个学生对应一个老师：关联

association：关联
collection：集合

方式一：按照查询嵌套查询

接口：StudentMapper.java

// 查询所有的学生和对应的老师信息
List<Student> getStudents();

mapper文件：StudentMapper.xml

<!--
思路：
1. 查询所有的学生信息
2. 根据查询出来学生信息的tid，查询对应的老师     （子查询）
-->
<select id="getStudents" resultMap="StudentTeacher">
 select * from student
</select>
<resultMap id="StudentTeacher" type="student">
 <!-- Property：实体属性；column：数据库字段-->
 <id property="id" column="id"/>
 <result property="name" column="name"/>
 <!-- 复杂类型需要单独处理 对象：association  集合：collection -->
 <association property="teacher" column="tid" javaType="teacher" select="getTeacher"/>
</resultMap>
<select id="getTeacher" resultType="teacher" parameterType="_int">
 select * from teacher where id = #{id}
</select>

核心配置文件中绑定注册StudentMapper

<!-- 给单独的实体类起别名 -->
<typeAliases>
 <package name="com.tassel.pojo"/>
</typeAliases>
<mappers>
 <!-- 每一个mapper配置文件都要在mybatis核心配置文件中注册 -->
 <mapper class="com.tassel.dao.TeacherMapper"/>
 <mapper class="com.tassel.dao.StudentMapper"/>
</mappers>

测试：

@Test
public void testStudent(){
 SqlSession sqlSession = MybatisUtil.getSqlSession();

 StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
 List<Student> students = mapper.getStudents();
 for (Student student : students) {
     System.out.println(student);
 }
 sqlSession.close();
}

8.Mybatis多(一)对一(多) - 图1

方式二：按照结果嵌套查询

 <!-- 按照结果嵌套查询 -->
<select id="getStudents2" 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">
    <id property="id" column="sid"/>
    <result property="name" column="sname"/>
    <!-- 这里mybatis 自动对应Teacher的属性-->
    <association property="teacher" javaType="teacher">
        <result property="name" column="tname"/>
    </association>
</resultMap>

8.Mybatis多(一)对一(多) - 图2

一对多处理

环境搭建

创建实体类

@Data
public class Student {
 private Integer id;
 private String name;
 private Integer tid;
}

@Data
public class Teacher {
    private Integer id;
    private String name;
    private List<Student> students;
}

一对多实例

案例：获取指定老师下的所有学生及老师的信息：一个老师对应多个学生：集合

association：关联
collection：集合

接口：TeacherMapper.java

Teacher getTeacher(@Param("id") int id);

mapper配置文件：TeacherMapper.xml ```xml



3. 
测试
```java
@Test
public void test(){
    SqlSession sqlSession = MybatisUtil.getSqlSession();
    TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
    Teacher teacher = mapper.getTeacher(1);
    System.out.println(teacher);
    sqlSession.close();
}

Teacher(id=1, name=李永乐, students=[Student(id=1, name=小明, tid=1), Student(id=2, name=小红, tid=1), Student(id=3, name=小张, tid=1), Student(id=4, name=小李, tid=1), Student(id=5, name=小王, tid=1)])
按照查询嵌套处理类似多对一（子查询）。

小结

association：关联【多对一】
collection：集合【一对多】
javaType：指定实体类中属性的类型
ofType：用来映射List或集合中的 pojo 类型（泛型中的约束类型）