MyBatis系列之关联查询(一对一)

我们以一个例子demo的形式，说明Mybatis的一对一关联查询

设计teachers表

classes表设置外键

classes表

classes表是课程表，teachers表是教师信息表

这里是一对一的关联关系，当然实际中一般不是这样的，一般可以是多对多，不过这里只是举例子

项目的结构，如图：

Classes.java:

package com.mybatis.entity;

import java.util.List;

public class Classes {

  private int id;
  
  private String name;
  
  private Teacher teacher;

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

  public void setStudents(List<Student> students) {
    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 Teacher getTeacher() {
    return teacher;
  }

  public void setTeacher(Teacher teacher) {
    this.teacher = teacher;
  }
  
  @Override
  public String toString() {
    return "Classes [id=" + id + ", name=" + name + "]";
  }
  
}

Teacher.java:

package com.mybatis.entity;

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

  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;
  }
  

}

这里以两种方式进行使用

classesMapper.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">
<!-- 
    为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名+sql映射文件名，这样就能够保证namespace的值是唯一的 
-->
<mapper namespace="com.mybatis.mapper.classesMapper">

<!-- 
    在select标签中编写查询的SQL语句， 设置select标签的id属性为findById，id属性值必须是唯一的，不能够重复
    parameterType可以在MyBatis的配置文件里设置，也可以直接用包名+类名
 -->
 
    <select id="findById" parameterType="int" resultMap="ClassesResultMap">
    select * from classes c,teachers t where c.teacher_id=t.t_id and c.c_id=#{id}
    </select>
    <resultMap type="Classes" id="ClassesResultMap">
        <id property="id" column="c_id" />
        <result property="name" column="c_name" />
        <association property="teacher" column="teacher_id" javaType="Teacher">
            <id property="id" column="t_id" />
            <result property="name" column="t_name" />
        </association>
    </resultMap>
    
    <select id="findById2" parameterType="int" resultMap="ClassesResultMap2">
    select * from classes where c_id=#{id}
    </select>
    <resultMap type="Classes" id="ClassesResultMap2">
        <id property="id" column="c_id" />
        <result property="name" column="c_name" />
        <association property="teacher" column="teacher_id" javaType="Teacher" select="getTeacherInfo">
        </association>
    </resultMap>
    <select id="getTeacherInfo" parameterType="int" resultType="Teacher">
    select t_id,t_name from teachers where t_id=#{id}
    </select>
</mapper>

然后对mybatis进行配置：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" 
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
   
   <!-- 数据库配置文件  --> 
    <properties resource="db.properties" />  
     
   <!-- 实体类,简称 -设置别名 -->
   <typeAliases>
      <typeAlias alias="User" type="com.mybatis.entity.User" />
      <typeAlias alias="Classes" type="com.mybatis.entity.Classes" />
      <typeAlias alias="Student" type="com.mybatis.entity.Student" />
      <typeAlias alias="Teacher" type="com.mybatis.entity.Teacher" />
   </typeAliases>

   
   <!-- 读取db.properties里的数据，进行数据库配置 -->
   <environments default="development">
      <environment id="development">
         <transactionManager type="JDBC" />
         <dataSource type="POOLED">
            <property name="driver" value="${driver}"/>
               <property name="url" value="${url}" />
               <property name="username" value="${username}"/>
               <property name="password" value="${password}"/>
         </dataSource>
      </environment>
   </environments>
   
   <!--  
      注册userMapper.xml文件 
   <mappers>
      <mapper resource="com/mybatis/mapper/userMapper.xml"/>
   </mappers>
   -->
   
   <mappers>
      <mapper class="com.mybatis.mapper.UserMapper"/>
      <mapper resource="com/mybatis/mapper/classesMapper.xml"/>
   </mappers>
   
</configuration>

db,properties:

driver=com.mysql.jdbc.Driver
url=jdbc\:mysql\://localhost\:3306/db_mysql?useUnicode\=true&characterEncoding\=utf-8
username=root
password=111

编写测试类：

String resource = "mybatis-config.xml";
    try{
      //加载Mybatis的配置文件
      Reader reader = Resources.getResourceAsReader(resource);
      //构建sqlSession的工厂
          SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader);
      //创建可以执行sql的SqlSession
          SqlSession session = sessionFactory.openSession();
      UserMapper mapper = session.getMapper(UserMapper.class);
      User user = mapper.findById(1);
          System.out.println(user);
          
          Classes c = session.selectOne("com.mybatis.mapper.classesMapper.findById1",1);
          System.out.println(c);
        
          
    }catch (Exception e) {
      e.printStackTrace();
    }