• 什么是多对一？

生活中常见多对一关系的例子，在学校你会发现你的班级只有一个，但是班级可以拥有多个学生，那么这就是一个典型的多对一关系，可以通过对象进行关系的表示，多对一的图解如下：

 环境需要的SQL脚本：

CREATE TABLE `studentclass` (
	`id` INT(10) NOT NULL,
	`classNumber` VARCHAr(30) DEFAULT NULL,
	PRIMARY KEY (`id`)
);

INSERT INTO studentclass(`id`,`classNumber`) VALUES (08090101,'计算机与科学1班');

CREATE TABLE `Universitystudent` (
	`id` INT(10) NOT NULL,
	`name` VARCHAr(30) DEFAULT NULL,
	`cid` INT(10) DEFAULT NULL,
	PRIMARY KEY (`id`),
	KEY `fkcid`(`cid`),
	CONSTRAINT `fkcid` FOREIGN KEY (`cid`) REFERENCES `studentclass` (`id`)
);

INSERT INTO `Universitystudent`(`id`,`name`,`cid`) VALUES (1,'李阳',08090101);
INSERT INTO `Universitystudent`(`id`,`name`,`cid`) VALUES (2,'程奕霖',08090101);
INSERT INTO `Universitystudent`(`id`,`name`,`cid`) VALUES (3,'谢雨萱',08090101);
INSERT INTO `Universitystudent`(`id`,`name`,`cid`) VALUES (4,'王二蛋',08090101);
INSERT INTO `Universitystudent`(`id`,`name`,`cid`) VALUES (5,'陈杰',08090101);

1.建和数据库相对于的映射类Universitystudent.java和Studentclass.java

Universitystudent.java

public class Universitystudent {
    private int id;
    private String name;
    private Studentclass studentclass;

    @Override
    public String toString() {
        return "Universitystudent{" +
                "id=" + id +
                ", name='" + name + ''' +
                ", studentStudentclass{" +
                "id=" + id +
                ", classNumber='" + classNumber + ''' +
                '}';
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getClassNumber() {
        return classNumber;
    }

    public void setClassNumber(String classNumber) {
        this.classNumber = classNumber;
    }
}

2.在建对应的Mapper接口和文件Mapper映射文件

UniversitystudentMapper接口

public interface UniversitystudentMapper {
    //需求：查询班级下所对于的学生,第一个是按照查询嵌套处理
    List findUniversityStudent();
    //这个是按照查询嵌套处理
    List findUniversityStudentTwo();
}

UniversitystudentMapper.xml映射文件

    
    
        SELECT * FROM universitystudent
    
    
    
        SELECT * FROM studentclass WHERe id = #{cid}
    
    
        
    
    
    
        SELECT
               us.id sid,us.`name` sname,sc.classNumber Class
        FROM
               studentclass sc,universitystudent us
        WHERe
               us.cid = sc.id;
    
    
        
        
        
            
        
    

3.测试代码

@Test
    public void testFindUniversityStudent() {
        MybatisUtils mybatisUtils = new MybatisUtils();
        //注意哈，我这样偷懒用的链式写法哈，正常写不要这样干哈
        mybatisUtils.
                getSqlSession().
                getMapper(UniversitystudentMapper.class).
                findUniversityStudent().
                forEach(System.out::println);
        mybatisUtils.
                getSqlSession().
                getMapper(UniversitystudentMapper.class).
                findUniversityStudentTwo().
                forEach(System.out::println);
    }

运行图

•  什么是一对多？

我们还是刚刚上面多对一的例子讲解，在学校你会发现有很多的学生，每个学生都只有一个班级，但是班级可以拥有多个学生，那么这就是一个典型的一对多关系，可以通过对象进行关系的表示，多对一的图解如下：

  环境需要的SQL脚本不变

1.我们需要改一下我们刚刚那两个实体类

Studentclass.java

public class Studentclass {
    private int id;
    private String classNumber;
    List universitystudents;

    @Override
    public String toString() {
        return "Studentclass{" +
                "id=" + id +
                ", classNumber='" + classNumber + ''' +
                ", universitystudents=" + universitystudents +
                '}';
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getClassNumber() {
        return classNumber;
    }

    public void setClassNumber(String classNumber) {
        this.classNumber = classNumber;
    }

    public List getUniversitystudents() {
        return universitystudents;
    }

    public void setUniversitystudents(List universitystudents) {
        this.universitystudents = universitystudents;
    }
}

Universitystudent.java

public class Universitystudent {
    private int id;
    private String name;
    private int cid;

    @Override
    public String toString() {
        return "Universitystudent{" +
                "id=" + id +
                ", name='" + name + ''' +
                ", cid=" + cid +
                '}';
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getCid() {
        return cid;
    }

    public void setCid(int cid) {
        this.cid = cid;
    }
}

3.我们再写StudentclassMapper.java的接口

public interface StudentClassMapper {
    //需求根据班级Id查询该班下的学生
    List byIdClass(@Param("id")int id);
}

4.我们写映射文件StudentclassMapper.xml

    
        SELECT * FROM studentclass WHERe id = #{id}
    
    
        
    
    
        SELECT * FROM universitystudent WHERe cid = #{id}
    
    
        SELECT
               sc.classNumber Class,
               us.id ucid,
               us.`name` ucname
        FROM
             studentclass sc,universitystudent us
        WHERe sc.id = us.cid AND sc.id = #{id}
    
    
        
        
            
            
        
    

5.测试代码

public void testByIdClass() {
        MybatisUtils mybatisUtils = new MybatisUtils();
        SqlSession sqlSession = mybatisUtils.getSqlSession();
        StudentClassMapper studentMapper = sqlSession.getMapper(StudentClassMapper.class);
        List studentClassesList = studentMapper.byIdClass(8090101);
        for (Studentclass studentclass : studentClassesList) {
            System.out.println(studentclass);
        }
        List studentClassesListTwo = studentMapper.byIdClassTwo(8090101);
        for (Studentclass studentclass : studentClassesListTwo) {
            System.out.println(studentclass);
        }
    }

测试图片：

 当然我讲解的时候采用了两种方法，按照结果嵌套处理只要你SQL比较好就可以用这个，不好的话采用嵌套查询吧