select * from
projectrecord pr
left join 
projects po
on po.pid=pr.pid
left join 
emp e
on e.empno = pr.empno
where pr.pid=1

<?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.msb.mapper.EmpMapper">
    <!--resultMap是一個映射表，對應的字段會對應數據表的字段
    -->
    <resultMap id="projectMap" type="projects">
        <id column="pid" property="pid"></id>
        <result column="pname" property="pname"></result>
        <result column="money" property="money"></result>
        <collection property="projectrecordList" ofType="projectrecord">
            <id property="pid" column="pid"></id>
            <id property="empno" column="empno"></id>
            <association property="emp" javaType="emp">
                <id property="empno" column="empno"></id>
                <result property="ename" column="ename"></result>
                <result property="job" column="job"></result>
                <result property="sal" column="sal"></result>
                <result property="hiredate" column="hiredate"></result>
                <result property="mgr" column="mgr"></result>
                <result property="comm" column="comm"></result>
                <result property="deptno" column="deptno"></result>
            </association>
        </collection>
    </resultMap>
    <!--Projects findPiddle(int pid);-->
    <select id="findPiddle" resultMap="projectMap">
        select *from
            emp e
                inner join
            projectrecord pr
            on pr.empno=e.empno
                inner join
            projects po
            on po.pid= pr.pid
        where pr.pid=#{pid}
    </select>
</mapper>

//mapper表
package com.msb.mapper;
import com.msb.pojo.Dept;
import com.msb.pojo.Emp;
import com.msb.pojo.Projects;
import java.util.List;
public interface EmpMapper {
    /**
     * 實現根據項目編號查看所有參與項目的人員信息
     * @param pid 傳入項目號
     * @return 返回一個project的對象
     */
    Projects findPiddle(int pid);
}

package com.msb.pojo;
//一對一關系引入
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.io.Serializable;
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Projectrecord implements Serializable {
    private int empno;
    private int pid;
    //pr和e是一對一的關系
    private Emp emp;
}

package com.msb.pojo;
//一對多關系實體類設計
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.io.Serializable;
import java.util.List;

@NoArgsConstructor
@AllArgsConstructor
@Data
public class Projects implements Serializable {
    private int pid;
    private String pname;
    private int money;
    //po和pr是一對多的關系
    List<Projectrecord> projectrecordList;
}

package com.msb.pojo;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.sql.Date;

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Emp {
    private Integer empno;
    private String ename;
    private String job;
    private Integer mgr;
    private Date hiredate;
    private  Integer sal;
    private Integer comm;
    private  Integer deptno;
    private Dept dept;
}

jdbc_driver=com.mysql.cj.jdbc.Driver
jdbc_url=jdbc:mysql://127.0.0.1:3306/testtable?useSSL=false&useUnicode=true&characterEncoding=UTF-8&serverTimezone=Asia/Shanghai
jdbc_username=root
jdbc_password=root

log4j.rootLogger=debug,stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.err
log4j.appender.stdout.layout=org.apache.log4j.SimpleLayout
log4j.appender.logfile=org.apache.log4j.FileAppender
log4j.appender.logfile.File=d:/msb.log
log4j.appender.logfile.layout=org.apache.log4j.PatternLayout
log4j.appender.logfile.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %l %F %p %m%n

<?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>
    <!--引入外部配置文件-->
    <properties resource="jdbc.properties"></properties>
    <!--這里是起的別名，前面是名，后面是路徑-->
    <!--<typeAliases>
        <typeAlias alias="dept" type="com.msb.pojo.Dept"/>
    </typeAliases>-->
    <!--包別名，用到時候調用名字小寫即可,就會掃描msb下的所以實體類，用的時候實體類名小寫-->
    <typeAliases>
        <package name="com.msb"/>
    </typeAliases>
    <environments default="development">
        <environment id="development">
            <!--   簡單使用了 JDBC 的提交和回滾設置  -->
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">

                <property name="driver" value="${jdbc_driver}"/>
                <property name="url" value="${jdbc_url}"/>
                <property name="username" value="${jdbc_username}"/>
                <property name="password" value="${jdbc_password}"/>
            </dataSource>
        </environment>
    </environments>
    <!--加載mapper映射文件-->
    <mappers>
        <package name="com.msb.mapper"/>
    </mappers>
</configuration>

import com.msb.mapper.EmpMapper;
import com.msb.pojo.Dept;
import com.msb.pojo.Emp;
import com.msb.pojo.Projectrecord;
import com.msb.pojo.Projects;
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 org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class Test1 {
    SqlSession sqlSession = null;
    EmpMapper mapper =null;

   @Before
   public void test1(){
       //首先做一個對象SqlSessionFactoryBuilder建立一個繪話
       SqlSessionFactoryBuilder ssfb = new SqlSessionFactoryBuilder();
       //有一個文本輸入的io流進行讀取操作
       InputStream stream = null;
       try {
           //這里的路徑直接會定位到配置文件classes下面；所以這個文件在次目錄下--編譯和
           //-圖紙；對數據庫文件進行讀取，獲取一個io流，由于配置文件在classes下面，直接寫文件名即可
           stream = Resources.getResourceAsStream("sqlMapConfig.xml");
       } catch (IOException e) {
           e.printStackTrace();
       }
       //build需要指向一個文件進行讀取出來--工廠
       SqlSessionFactory factory = ssfb.build(stream);
        //需要用sqlSession去調用增刪改查--工人去獲取數據，打開這個繪話
       sqlSession = factory.openSession();
   }
    
    /*多對多的查詢*/
    @Test
    public void test7(){
        mapper = sqlSession.getMapper(EmpMapper.class);
        Projects piddle = mapper.findPiddle(1);
        System.out.println("Pid");
        System.out.println(piddle.getPid());
        System.out.println("Pname");
        System.out.println(piddle.getPname());
        System.out.println("Money");
        System.out.println(piddle.getMoney());
        System.out.println("ProjectrecordList");
        List<Projectrecord> projectrecordList = piddle.getProjectrecordList();
        projectrecordList.forEach(System.out::println);

    }
    
    @After
    public void test3(){
       if (sqlSession!=null){
           sqlSession.close();
       }
    }
}