使用Mybatis实现多表多对多查询实例(额外SQL方式以及关联查询方式含代码及解释)

多对多查询实例(查询学生)

多对多查询的时候,一定不要忘了关系表,不管增删改查都不要忘了关系表

项目目录结构(maven):

多对多查询生灵魂之处解析:

关联查询StudentMapper.xml

额外SQL查询StudentMapper.xml

额外SQL查询TeacherMapper.xml

(所有表id都设置了自动递增)

创建表t_stu:

创建表t_teacher:

 

创建关系表t_stu_tea:

在Maven配置文件pom.xml中添加所需要的依赖:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.xiongluoluo</groupId>
    <artifactId>manytomany</artifactId>
    <version>1.0.0</version>

    <dependencies>
        <!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.8</version>
            <scope>provided</scope>
        </dependency>

        <!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.4.6</version>
        </dependency>

        <!-- https://mvnrepository.com/artifact/junit/junit -->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.12</version>
            <scope>test</scope>
        </dependency>

        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.38</version>
        </dependency>

        <!--日志包-->
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.7.25</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
            <version>1.7.25</version>
            <scope>test</scope>
        </dependency>

    </dependencies>

</project>

创建实体类Student:

package com.xiongluoluo.bean;

import lombok.*;

import java.util.List;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
@Getter
@Setter
@ToString
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private int id;
    private String name;
    private List<Teacher> teacherList;
    /*这里new不new对象都行*/
}

创建实体类Teacher:

package com.xiongluoluo.bean;

import lombok.*;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
@Getter
@Setter
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class Teacher {
    private int id;
    private String name;
}

创建mybatis配置文件Mybatis.xml:

<?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>
    <properties resource="db.properties"/>
    <!--这里用的是resource属性,导入外部properties文件-->

    <typeAliases>
        <package name="com.xiongluoluo.bean"/>
    </typeAliases>
    
    <environments default="mysql">
        <environment id="mysql">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driverClassName}"/>
                <!--这里是driver-->
                <!--driverClassName是properties文件中的变量,通过${}取其对应的支持-->
                <!--下面同理-->
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>

    <mappers>
        <mapper resource="com/xiongluoluo/mapper/StudentMapper.xml"/>

        <mapper resource="com/xiongluoluo/mapper/TeacherMapper.xml"/>
        <!--这里是斜线(因为是目录),不是点,不是点,不是点,你能不能不要在一个错误上躺平,每天忘提交事务,每天写...,唉-->
        <!--Mybatis的配置文件,也就是这个文件,是程序的入口.我们配置的mapper文件如果不在这里注册
        那么程序是读不到的.所以一定别忘记在这里添加.
        这里用的也是resource属性,引入外部的mapper.xml属性-->
    </mappers>
</configuration>

db.properties文件:

driverClassName=com.mysql.jdbc.Driver
url=jdbc:mysql:///mydb1228
username=root
password=1234

log4j.properties文件:

# Global logging configuration
log4j.rootLogger=DEBUG, stdout
# MyBatis logging configuration...
log4j.logger.com.xiongluoluo.mapper=TRACE
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

创建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.xiongluoluo.mapper.StudentMapper">
  

<!-- 关联查询

多对多通过id查询学生信息,属性是list用collection,属性是单个对象,用association
 <resultMap id="studentResultMap" type="Student">
     <id property="id" column="sid"/>
     <result property="name" column="sname"/>
     <collection property="teacherList" ofType="Teacher">
         <id property="id" column="id"/>
         <result property="name" column="name"/>
     </collection>
 </resultMap>

 <select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
     select s.id sid,s.name sname,t.id,t.name from t_stu s,t_teacher t,t_stu_tea sc where sc.stu_id=s.id and sc.tea_id=t.id and s.id=#{id}
 </select>-->



 <!--额外SQL查询-->
 <resultMap id="studentResultMap" type="Student">
     <id property="id" column="id"/>
     <result property="name" column="name"/>
     <collection property="teacherList" ofType="Teacher" column="id"
                 select="com.xiongluoluo.mapper.TeacherMapper.selectTeacherByStudentId"/>

 </resultMap>
 <select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
     select id,name from t_stu where id=#{id}
 </select>
</mapper>

创建TeacherMapper.xml

package com.xiongluoluo.mapper;

import com.xiongluoluo.bean.Teacher;

import java.util.List;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
public interface TeacherMapper {
    public List<Teacher> selectTeacherByStudentId(int Stu_id);
}

 

创建StudentMapper.java:

package com.xiongluoluo.mapper;

import com.xiongluoluo.bean.Student;
import org.apache.ibatis.annotations.Param;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
public interface StudentMapper {
   

/*通过id查询学生信息*/
public Student selectStudentById(int id);
}

创建TeacherMapper.java:

package com.xiongluoluo.mapper;

import com.xiongluoluo.bean.Teacher;

import java.util.List;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
public interface TeacherMapper {
    public List<Teacher> selectTeacherByStudentId(int Stu_id);
}

 

创建StudentService.java:

package com.xiongluoluo.service;

import com.xiongluoluo.bean.Student;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
public interface StudentService {
  

/*通过id查询学生信息*/
public Student findStudentById(int id);
}

创建TeacherService.java:

package com.xiongluoluo.service;

import com.xiongluoluo.bean.Teacher;

import java.util.List;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
public interface TeacherService {
    public List<Teacher> findTeacherByStudentId(int Stu_id);
}

 

创建StudentServiceImpl.java:

package com.xiongluoluo.service.impl;

import com.xiongluoluo.bean.Student;
import com.xiongluoluo.bean.Teacher;
import com.xiongluoluo.mapper.StudentMapper;
import com.xiongluoluo.service.StudentService;
import com.xiongluoluo.utils.MybatisUtil;
import org.apache.ibatis.session.SqlSession;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
public class StudentServiceImpl implements StudentService {
   

/*通过id查询学生信息*/
public Student findStudentById(int id) {
    SqlSession sqlSession = MybatisUtil.getSqlSession();
    StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
    Student student = mapper.selectStudentById(id);
    sqlSession.commit();
    sqlSession.close();
    //查询可以不提交事务,增删改一定要记得提交事务
    return student;
    //后返回
}
}

创建TeacherServiceImpl.java:

package com.xiongluoluo.service.impl;

import com.xiongluoluo.bean.Teacher;
import com.xiongluoluo.mapper.TeacherMapper;
import com.xiongluoluo.service.TeacherService;
import com.xiongluoluo.utils.MybatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.util.List;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
public class TeacherServiceImpl implements TeacherService {
    public List<Teacher> findTeacherByStudentId(int stu_id) {
        SqlSession sqlSession = MybatisUtil.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        List<Teacher> teachers = mapper.selectTeacherByStudentId(stu_id);
        sqlSession.commit();
        sqlSession.close();
        return teachers;
    }
}

 

获取SqlSession的工具类:

package com.xiongluoluo.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;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
public class MybatisUtil {
    private static SqlSessionFactory sqlSessionFactory = null;
    static {
        try {
            InputStream is = Resources.getResourceAsStream("Mybatis.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }
}

 

创建测试类:

package com.xiongluoluo.test;

import com.xiongluoluo.bean.Student;
import com.xiongluoluo.service.StudentService;
import com.xiongluoluo.service.impl.StudentServiceImpl;
import org.junit.Test;

/**
 * Created by Administrator on 2019/12/28 0028.
 */
public class MybatisTest {
 

    @Test
    public void test2(){

        StudentService studentService = new StudentServiceImpl();
        Student student = studentService.findStudentById(1);
        System.out.println(student);
        System.out.println(student.getTeacherList());
    }
}