mybatis中多对一查询

实体类：（注：这里使用了lombok插件）

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

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

实体类对应的接口类：

public interface StudentMapper {
    /**
     * 获取所有学生的信息，包括对应的老师信息
     *
     * @return
     */
    public List<Student> getStudents();
}

public interface TeacherMapper {
    /**
     * 根据老师id来查询老师信息
     *
     * @param id
     * @return
     */
    @Select("select * from teacher where id=#{tid}")
    Teacher getTeacherById(@Param("tid") int id);
}

需求：

1. 查询出所有的学生信息，其中他的老师信息也一起查出来。

   • 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.luwanglin.dao.StudentMapper">
         <select id="getStudents" resultMap="StudentTeacher">
             select * from student
         </select>
         <resultMap id="StudentTeacher" type="Student">
             <id property="id" column="id"/>
             <result property="name" column="name"/>
             <association property="teacher" column="{demo=tid,testname=name}" javaType="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。
     
         个人测试：
         column只有一个属性时，可以在下面的getTeacher中任意名字取出值，有多个时以键值对的形式传递
       -->
         <select id="getTeacher" resultType="teacher">
             select * from teacher where id=#{demo};
         </select>
     </mapper>
     
     

     注：记得注册mapper

   • 编写测试类：

     package com.luwanglin.dao;
     
     import com.luwanglin.pojo.Student;
     import com.luwanglin.pojo.Teacher;
     import com.luwanglin.utils.MybatisUtils;
     import org.apache.ibatis.session.SqlSession;
     import org.junit.jupiter.api.AfterEach;
     import org.junit.jupiter.api.BeforeEach;
     import org.junit.jupiter.api.Test;
     
     import java.util.List;
     
     /**
      * @author luwanglin
      * @email 1769862620@qq.com
      * @Date 2020/9/15 10:00
      * @Version 1.0
      */
     public class UserMapperTest {
         SqlSession sqlSession;
     
         @Test
         void testGetTeacherById() {
             TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
             Teacher teacher = mapper.getTeacherById(1);
             System.out.println(teacher);
         }
     
         @Test
         void testGetStudents() {
             StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
             List<Student> students = mapper.getStudents();
             for (Student student : students) {
                 System.out.println(student);
             }
         }
     
     
         @BeforeEach
         public void initSqlSession() {
     //        System.out.println("这是BeforeEach！");
             //MybatisUtils是一个工具类，用来获取sqlsession
             sqlSession = MybatisUtils.getSession();
         }
     
         @AfterEach
         void closeSqlSession() {
             sqlSession.close();
         }
     }
     

     工具类：

     package com.luwanglin.utils;
     
     import org.apache.ibatis.io.Resources;
     import org.apache.ibatis.session.SqlSession;
     import org.apache.ibatis.session.SqlSessionFactory;
     import org.apache.ibatis.session.SqlSessionFactoryBuilder;
     
     import java.io.IOException;
     import java.io.InputStream;
     
     /**
      * @author luwanglin
      * @email 1769862620@qq.com
      * @Date 2020/9/15 9:09
      * @Version 1.0
      */
     public class MybatisUtils {
         private static SqlSessionFactory sqlSessionFactory = null;
     
         static {
             try {
                 String resource = "mybatis-config.xml";
                 InputStream inputStream = Resources.getResourceAsStream(resource);
                 sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
             } catch (IOException e) {
                 e.printStackTrace();
             }
         }
     
         public static SqlSession getSession() {
             return sqlSessionFactory.openSession();
         }
     
     }
     

   • 结果如下：

     Student(id=1, name=小明, teacher=Teacher(id=1, name=秦老师))
     Student(id=2, name=小红, teacher=Teacher(id=1, name=秦老师))
     Student(id=3, name=小张, teacher=Teacher(id=1, name=秦老师))
     Student(id=4, name=小李, teacher=Teacher(id=1, name=秦老师))
     Student(id=5, name=小王, teacher=Teacher(id=1, name=秦老师))
     Student(id=6, name=小哈, teacher=Teacher(id=1, name=秦老师))
     

   为了查看传递键值对的效果，这里采用了如下测试：

   <select id="getTeacher" resultType="teacher">
       select * from teacher where id=#{demo} or name=#{testname};
   /*
   这里就根据上面的键值对取出了demo对应于tid
   */
   </select>
   

   这里同样是可以查出学生的信息以及对应的老师信息。此时控制台打印的日志信息如下：

   2020-09-17 10:12:48 [com.luwanglin.dao.StudentMapper.getStudents:137]-[DEBUG] ==>  Preparing: select * from student
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getStudents:137]-[DEBUG] ==> Parameters: 
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====>  Preparing: select * from teacher where id=? or name=?; /* 这里就根据上面的键值对取出了demo对应于tid */
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====> Parameters: 1(Integer), 小明(String)
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] <====      Total: 1
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====>  Preparing: select * from teacher where id=? or name=?; /* 这里就根据上面的键值对取出了demo对应于tid */
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====> Parameters: 1(Integer), 小红(String)
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] <====      Total: 1
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====>  Preparing: select * from teacher where id=? or name=?; /* 这里就根据上面的键值对取出了demo对应于tid */
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====> Parameters: 1(Integer), 小张(String)
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] <====      Total: 1
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====>  Preparing: select * from teacher where id=? or name=?; /* 这里就根据上面的键值对取出了demo对应于tid */
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====> Parameters: 1(Integer), 小李(String)
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] <====      Total: 1
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====>  Preparing: select * from teacher where id=? or name=?; /* 这里就根据上面的键值对取出了demo对应于tid */
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====> Parameters: 1(Integer), 小王(String)
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] <====      Total: 1
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====>  Preparing: select * from teacher where id=? or name=?; /* 这里就根据上面的键值对取出了demo对应于tid */
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] ====> Parameters: 1(Integer), 小哈(String)
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getTeacher:137]-[DEBUG] <====      Total: 1
   2020-09-17 10:12:49 [com.luwanglin.dao.StudentMapper.getStudents:137]-[DEBUG] <==      Total: 6
   Student(id=1, name=小明, teacher=Teacher(id=1, name=秦老师))
   Student(id=2, name=小红, teacher=Teacher(id=1, name=秦老师))
   Student(id=3, name=小张, teacher=Teacher(id=1, name=秦老师))
   Student(id=4, name=小李, teacher=Teacher(id=1, name=秦老师))
   Student(id=5, name=小王, teacher=Teacher(id=1, name=秦老师))
   Student(id=6, name=小哈, teacher=Teacher(id=1, name=秦老师))
   

   可以从日志中看出这里的确采用的是嵌套查询，因为每一条记录都进行了一次嵌套查询。

   而且每次查询的条件是根据传递的键值对来进行取值传入的。