Mybatis 实现多对一和一对多的数据库操作

Mybatis 实现多对一和一对多的数据库操作

第一步：建立对应数据库表：

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`) VALUES (1, '傅老师'); 

建立一个老师表并插入一个老师（注意创建和插入要分开执行）

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');

建立一个学生表并插入五个学生（注意创建和插入要分开执行）

第二步：建立pojo实体类：

Teacher:

package com.xiaofu.pojo;

public class Teacher {
    private int id;
    private String name;
    //无参构造
    public Teacher() {
    }
    //有参构造
    public Teacher(int id, String name) {
        this.id = id;
        this.name = name;
    }

    //get set 方法
    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;
    }
    
    //重写toString
    @Override
    public String toString() {
        return "Teacher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

 Student:

package com.xiaofu.pojo;

public class Student {
    private int id;
    private String name;
    //学生需要关联一个老师 所以这里封装一个Teacher类型
    private Teacher teacher;
    //无参构造
    public Student() {
    }
    //有参构造
    public Student(int id, String name, Teacher teacher) {
        this.id = id;
        this.name = name;
        this.teacher = teacher;
    }
    //get set 方法
    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 Teacher getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
    //重写toString
    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", teacher=" + teacher +
                '}';
    }
}

第三步：建立接口：

第四步：建立实现接口的Mapper.xml：

 第五步：在核心配置文件中注册 写好的Mapper.xml:

 

 第六步：实现多对一：

package com.xiaofu.dao;
import com.xiaofu.pojo.Student;
import java.util.List;

public interface StudentMapper {
    //查询所有的学生信息及对应老师的信息（多对一）
    List<Student> getStudent();
}

在StudentMapper中编写一个接口来完成多对一的查询

 然后在StudentMapper.xml中实现接口：

第一种查询方式：按照查询嵌套处理：

<?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.xiaofu.dao.StudentMapper">
<!--    查询所有的学生信息及对应老师的信息（多对一）-->
<!--    第一种解决方式：按照查询嵌套处理-->
<!--    先查出所有的学生-->
    <select id="getStudent" resultMap="StudentTeacher">
        select * from student
    </select>
<!-- resultMap 结果集映射 这里的id 对应上面的resultMap   -->
    <resultMap id="StudentTeacher" type="com.xiaofu.pojo.Student">
        <result property="id" column="id"/> <!--模型类中的id（property）对应查出来的id（column）-->
        <result property="name" column="name"/><!--模型类中的name（property）对应查出来的name（column）-->
        <!--模型类中的teacher对应下面的查询语句  -->
        <!--为什么呢？因为column="tid" 被传到了下面查询语句的#{tid} javaType 确定了字段的类型 select="getTeacher" 执行了下面的查询语句 -->
        <association property="teacher" column="tid" javaType="com.xiaofu.pojo.Teacher" select="getTeacher"/>
    </resultMap>
    <select id="getTeacher" resultType="com.xiaofu.pojo.Teacher">
        select * from teacher where id = #{tid}
    </select>

</mapper>

第二种查询方式：按照结果嵌套处理：

<?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.xiaofu.dao.StudentMapper">
<!--    查询所有的学生信息及对应老师的信息（多对一）-->
<!--    第二种解决方式：按照结果嵌套处理-->
    <select id="getStudent" resultMap="StudentTeacher">
        select s.id sid,s.name sname,t.name tname
        from student s,teacher t
        where s.tid = t.id
    </select>
<!-- resultMap 结果集映射 这里的id 对应上面的resultMap   -->
    <resultMap id="StudentTeacher" type="com.xiaofu.pojo.Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <!-- 因为teacher对应的是一个对象 所以在映射对应的内容   -->
            <association property="teacher" javaType="com.xiaofu.pojo.Teacher">
                <result property="name" column="tname"/>
            </association>
    </resultMap>

</mapper>

编写测试类：

    @Test
    public void getlist(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);


        List<Student> list = mapper.getStudent();

        for (Student student : list) {
            System.out.println(student);
        }
        sqlSession.close();
    }

分别执行一下：

 都查了出来。

 第七步：一对多：

做一对多需要先更改一下实体类：

Sutdent：

package com.xiaofu.pojo;

public class Student {
    private int id;
    private String name;
    //学生需要关联一个老师 所以这里封装一个Teacher类型
    private int tid;

    public Student() {
    }

    public Student(int id, String name, int tid) {
        this.id = id;
        this.name = name;
        this.tid = tid;
    }

    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 getTid() {
        return tid;
    }

    public void setTid(int tid) {
        this.tid = tid;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", tid=" + tid +
                '}';
    }
}

Teacher：

package com.xiaofu.pojo;

import java.util.List;

public class Teacher {
    private int id;
    private String name;
    //一个老师拥有多个学生
    private List<Student> students;

    public Teacher() {
    }

    public Teacher(int id, String name, List<Student> students) {
        this.id = id;
        this.name = name;
        this.students = students;
    }

    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 List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }
    @Override
    public String toString() {
        return "Teacher{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", students=" + students +
                '}';
    }
}

 在TecherMapper下写一个接口：

package com.xiaofu.dao;

import com.xiaofu.pojo.Teacher;

public interface TeacherMapper {

    //获取指定老师下的所有学生（及老师的信息和学生的信息）
    Teacher getTeacher(int id);
}

 在TeacherMapper.xml中实现方法：

第一种方式：按照结果嵌套处理：

<?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.xiaofu.dao.TeacherMapper">
        <select id="getTeacher" resultMap="TeacherStudent">
            select s.id sid, s.name sname,t.name tname,t.id tid
            from student s, teacher t
            where s.tid = t.id and t.id = #{tid}
        </select>
        <resultMap id="TeacherStudent" type="com.xiaofu.pojo.Teacher">
            <result property="id" column="tid"/>
            <result property="name" column="tname"/>
           <!--复杂的属性，我们需要单独处理 对象：association 集合：collection
           javaType=“ ” 指定属性类型！
           集合中的泛型信息，我们使用ofType获取
           -->
            <collection property="students" ofType="com.xiaofu.pojo.Student">
                <result property="id" column="sid"/>
                <result property="name" column="sname"/>
                <result property="tid" column="tid"/>
            </collection>
        </resultMap>
</mapper>

第二种查询方式：按照查询嵌套处理：

<?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.xiaofu.dao.TeacherMapper">
    <select id="getTeacher" resultMap="TeacherStudent">
        select * from teacher where id = #{tid}
    </select>

    <resultMap id="TeacherStudent" type="com.xiaofu.pojo.Teacher">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <collection property="students" column="id" javaType="ArrayList" ofType="Student" select="getStudent"/>
    </resultMap>

    <select id="getStudent" resultType="com.xiaofu.pojo.Student">
        select * from student where tid = #{tid};
    </select>


</mapper>

编写测试类：

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


        sqlSession.close();
    }

分别执行一下：

 都查出来了