MyBatis中多对一和一对多数据的处理-摩杜云开发者社区

多对一的处理

多对一的理解：

多个学生对应一个老师
如果对于学生这边，就是一个多对一的现象，即从学生这边关联一个老师！

1、数据库设计

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, '吴老师'); 

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');

2、搭建测试环境

【Lombok的使用】

1. IDEA安装Lombok插件

2. 引入Maven依赖

<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <version>1.16.10</version>
</dependency>

3. 在代码中增加注解

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

@Data
public class Student {
    private int id;
    private String name;
    //多个学生可以是同一个老师，即多对一
    private Teacher teacher;
}

4. 编写实体类对应的Mapper接口【两个】

无论有没有需求，都应该写上，以备后来之需！

public interface StudentMapper {
}

public interface TeacherMapper {
}

5. 编写Mapper接口对应的 mapper.xml配置文件【两个】

无论有没有需求，都应该写上，以备后来之需！

<?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.hang.mapper.StudentMapper">
</mapper>

<?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.hang.mapper.TeacherMapper">
</mapper>

3、按查询嵌套处理

1. 给StudentMapper接口增加方法

//获取所有学生及对应老师的信息
public List<Student> getStudents();

2. 编写对应的Mapper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.hang.mapper.StudentMapper">

    <select id="getStudents" resultMap="studentTeacher">
        select * from mybatis.student;
    </select>

    <resultMap id="studentTeacher" type="student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--association关联属性 property属性名 javaType属性类型 column在多的一方的表中的列名-->
        <association property="teacher" column="tid" javaType="teacher" select="getTeacher"/>
    </resultMap>

    <select id="getTeacher" resultType="teacher">
        select * from mybatis.teacher;
    </select>

</mapper>

3. 编写完毕去Mybatis配置文件中，注册Mapper！

4. 测试

    @Test
    public void getStudents(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> students= mapper.getStudents();
        for (Student student : students) {
            System.out.println(
            "学生名:"+ student.getName()
            +"\t老师:"+student.getTeacher().getName());
        }
        sqlSession.close();
    }

4 、按结果嵌套处理

除了上面这种方式，还有其他思路吗？

我们还可以按照结果进行嵌套处理；

1. 接口方法编写

public List<Student> getStudents2();

2. 编写对应的mapper文件

　　<select id="getStudents2" resultMap="studentTeacher2">
        select s.id sid, s.name sname, t.id tid, t.name tname
        from mybatis.student s, mybatis.teacher t
        where t.id = s.tid
    </select>

    <resultMap id="studentTeacher2" type="student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="teacher">
            <result property="id" column="tid"/>
            <result property="name" column="tname"/>
        </association>
    </resultMap>

3. 去mybatis-config文件中注入【此处应该处理过了】

4. 测试

    @Test
    public void getStudents2(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> students= mapper.getStudents2();
        for (Student student : students) {
            System.out.println(
            "学生名:"+ student.getName()
            +"\t老师:"+student.getTeacher().getName());
        }
        sqlSession.close();
    }

5、小结

按照查询进行嵌套处理就像SQL中的子查询
按照结果进行嵌套处理就像SQL中的联表查询

一对多的处理

一对多的理解：

一个老师拥有多个学生
如果对于老师这边，就是一个一对多的现象，即从一个老师下面拥有一群学生（集合）！

1、实体类编写

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

@Data
public class Teacher {
    private int id;
    private String name;
    //一个老师多个学生
    private List<Student> students;
}

和之前一样，搭建测试的环境！

2、按结果嵌套处理

1. TeacherMapper接口编写方法

//获取指定老师，及老师下的所有学生
public Teacher getTeacher(int id);

2. 编写接口对应的Mapper配置文件

    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid, s.name sname, t.id tid, t.name tname
        from mybatis.teacher t, mybatis.student s
        where t.id = s.tid 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>

3. 将Mapper文件注册到MyBatis-config文件中

4. 测试

@Test
public void testGetTeacher(){
    SqlSession session = MybatisUtils.getSession();
    TeacherMapper mapper = session.getMapper(TeacherMapper.class);
    Teacher teacher = mapper.getTeacher(1);
    System.out.println(teacher.getName());
    System.out.println(teacher.getStudents());
}

3、按查询嵌套处理

1. TeacherMapper接口编写方法

public Teacher getTeacher2(int id);

2. 编写接口对应的Mapper配置文件

    <select id="getTeacher2" resultMap="TeacherStudent2">
        select * from teacher where id = #{id}
    </select>
    <resultMap id="TeacherStudent2" type="Teacher">
        <!--column是一对多的外键 , 写的是一的主键的列名-->
        <collection property="students" javaType="ArrayList" ofType="Student" column="id" select="getStudentByTeacherId"/>
    </resultMap>
    <select id="getStudentByTeacherId" resultType="Student">
        select * from student where tid = #{id}
    </select>

3. 将Mapper文件注册到MyBatis-config文件中

4. 测试

@Test
public void testGetTeacher2(){
    SqlSession session = MybatisUtils.getSession();
    TeacherMapper mapper = session.getMapper(TeacherMapper.class);
    Teacher teacher = mapper.getTeacher2(1);
    System.out.println(teacher.getName());
    System.out.println(teacher.getStudents());
}

4、小结

关联-association
集合-collection
所以association是用于一对一和多对一，而collection是用于一对多的关系
JavaType和ofType都是用来指定对象类型的

- JavaType是用来指定pojo中属性的类型
- ofType指定的是映射到list集合属性中pojo的类型。

注意说明：

保证SQL的可读性，尽量通俗易懂
根据实际要求，尽量编写性能更高的SQL语句
注意属性名和字段不一致的问题
注意一对多和多对一中：字段和属性对应的问题
尽量使用Log4j，通过日志来查看自己的错误