MyBatis 04 多对多的关系

1. 数据准备

数据库

student 数据库字段：

• id int(10)
• name varchar(30)
• tid int(10)

teacher 数据库字符：

• id int(10)
• name varchar(30)

实体类

Student:

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

Teacher:

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

SQL语句

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, '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');

实体接口

编写实体类对应的Mapper接口 【两个】

public interface StudentMapper {
}
public interface TeacherMapper {
}

mapper.xml

编写Mapper接口对应的 mapper.xml配置文件
StudentMapper.xml:

<?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.kuang.mapper.StudentMapper">

</mapper>

TeacherMapper.xml:

<?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.kuang.mapper.TeacherMapper">

</mapper>

2.一对多的关系

多个学生对应一个老师.

2.1 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:

<mapper namespace="com.dao.StudentMapper">
<!--
   需求：获取所有学生及对应老师的信息
   思路：
       1. 获取所有学生的信息
       2. 根据获取的学生信息的老师ID->获取该老师的信息
       3. 思考问题，这样学生的结果集中应该包含老师，该如何处理呢，数据库中我们一般使用关联查询？
           1. 做一个结果集映射：StudentTeacher
           2. StudentTeacher结果集的类型为 Student
           3. 学生中老师的属性为teacher，对应数据库中为tid。
              多个 [1,...）学生关联一个老师=> 一对一，一对多
           4. 查看官网找到：association – 一个复杂类型的关联；使用它来处理关联查询
   -->
    <select id="getStudents" resultMap="studentAllInfo" >
        select * from student
    </select>

    <resultMap id="studentAllInfo" type="com.pojo.Student">
        <result column="id" property="id" />
        <result column="name" property="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="com.pojo.Teacher" select="getTeacher" />
    </resultMap>
 <!--
   这里传递过来的id，只有一个属性的时候，下面可以写任何值
   association中column多参数配置：
       column="{key=value,key=value}"
       其实就是键值对的形式，key是传给下个sql的取值名称，value是片段一中sql查询的字段名。
   -->
        <select id="getTeacher" resultType="com.pojo.Teacher">
        select * from teacher where id = #{tid}
    </select>
</mapper>

2.2 按结果嵌套处理

1. 接口方法编写:
   public List<Student> getStudents2();
2. StudentMapper.xml:

    <select id="getStudents2" resultMap="STInfo">
        select s.id sid, s.name sname, t.id tid, t.name tname
        from student s, teacher t
        where s.tid = t.id
    </select>
    <resultMap id="STInfo" type="com.pojo.Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname" />
        <association property="teacher" javaType="com.pojo.Teacher">
            <result property="id" column="tid" />
            <result property="name" column="tname" />
        </association>
    </resultMap>

Test:

    @Test
    public void testStudent01(){
        SqlSession sqlsession = MyBatisUtils.getSqlSession();
        StudentMapper studentMapper = sqlsession.getMapper(StudentMapper.class);
        List<Student> listStudent = studentMapper.getStudents2();
        for (Student student : listStudent) {
            System.out.println(student);
        }
    }

3. 多对一的关系

一个老师对应多个学生

数据准备

Student

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

Teacher

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

    // 一个老师拥有多个学生
    private List<Student> studentList;
}

3.1 按结果嵌套处理

Mapper接口

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

<mapper namespace="com.dao.TeacherMapper">
    
   <!--
   思路:
       1. 从学生表和老师表中查出学生id，学生姓名，老师姓名
       2. 对查询出来的操作做结果集映射
          * 集合的话，使用collection！
          * 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。
   -->

    <select id="getTeacher" resultMap="TeacherStuent" >
        select s.id sid, s.name sname, t.id tid, t.name tname  from student s, teacher t
        where s.tid = t.id and t.id = #{tid}
    </select>

    <resultMap id="TeacherStuent" type="com.pojo.Teacher">
        <result property="id" column="tid" />
        <result property="name" column="tname" />
        <!--集合的话，使用collection
            property : 对应着 Teacher类中的属性；
            javaType: 指定属性的类型；
            集合中的泛型，使用ofType获取
        ！-->
        <collection property="studentList" ofType="com.pojo.Student">
            <result property="id" column="sid" />
            <result property="name" column="sname" />
            <result property="tid" column="tid" />
        </collection>

    </resultMap>
</mapper>

Test:

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

3.2 按查询嵌套处理

数据准备

@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;
}

1. TeacherMapper接口编写方法 :
   public Teacher getTeacher2(int id);
2. 2、编写接口对应的Mapper配置文件

<mapper namespace="com.dao.TeacherMapper">
    <select id="getTeacher2" resultMap="TeacherStuent">
        select * from teacher where id = #{tid}
    </select>

    <resultMap id="TeacherStuent" type="com.pojo.Teacher">
        <result property="id" column="id" />
        <result property="name" column="name" />
        <!--column是一对多的外键 , 写的是一的主键的列名-->
        <collection property="studentList" javaType="ArrayList" ofType="com.pojo.Student" select="getStudentByTId" column="id"/>
    </resultMap>
    
    <select id="getStudentByTId" resultType="com.pojo.Student">
        select * from student where tid = #{id}
    </select>
</mapper>

Test:

    @Test
    public void testgetTeacher2(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = teacherMapper.getTeacher2(1);
        System.out.println(teacher);

        sqlSession.close();

    }