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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：

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。

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原文地址：https://www.cnblogs.com/luckforefforts/p/13684424.html