一对多级联查询

在实际开发中经常会遇到一对多查询，比如说一个班级对应多个学生，在需求中我们又需要查询出每个班的学生。
首先先准备数据表
直接运行下面SQL语句即可

create database boss;

use boss;

create table meeting
(
    id    int auto_increment primary key,
    theme varchar(255)
);
create table boss
(
    id         int auto_increment primary key,
    name       varchar(255),
    company    varchar(255),
    meeting_id int
);
create table car
(
    id        int auto_increment primary key,
    brand     varchar(255),
    color     varchar(255),
    parameter varchar(255)
);
insert into meeting(theme)
values ('四大家族会议');
insert into car(brand, color, parameter)
values ('兰博基尼', '蓝色', '2000万-5000万');
insert into car(brand, color, parameter)
values ('法拉利','红色','500万-2000万');
insert into car(brand, color, parameter)
values('迈凯伦','黑色','500万-2000万');
insert into car(brand, color, parameter)
values ('布加迪','灰色','500万-2000万');
insert into boss(name, company, meeting_id)
values ('小赵','赵氏家族',1);
insert into boss(name, company, meeting_id)
values ('小王','王氏家族',1);
insert into boss(name, company, meeting_id)
values ('小李','李氏家族',1);
insert into boss(name, company, meeting_id)
values ('小陆','陆氏家族',1);

实体类
Meeting类

package cn.zl.jdbc3.entity;


import java.util.List;

public class Meeting {

    private long id;
    private String theme;
    private List<Boss> bossList;

    public List<Boss> getBossList() {
        return bossList;
    }

    public void setBossList(List<Boss> bossList) {
        this.bossList = bossList;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }


    public String getTheme() {
        return theme;
    }

    public void setTheme(String theme) {
        this.theme = theme;
    }

    @Override
    public String toString() {
        return "Meeting{" +
                "id=" + id +
                ", theme='" + theme + '\'' +
                "\n, bossList=" + bossList +
                '}';
    }
}

Boss类

package cn.zl.jdbc3.entity;


public class Boss {

    private long id;
    private String name;
    private String company;
    private Car car;

    public Car getCar() {
        return car;
    }

    public void setCar(Car car) {
        this.car = car;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }


    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


    public String getCompany() {
        return company;
    }

    public void setCompany(String company) {
        this.company = company;
    }

    @Override
    public String toString() {
        return "Boss{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", company='" + company + '\'' +
                ", car=" + car +
                '}';
    }
}

Car类

package cn.zl.jdbc3.entity;


public class Car {

    private long id;
    private String brand;
    private String color;
    private String parameter;


    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }


    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }


    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }


    public String getParameter() {
        return parameter;
    }

    public void setParameter(String parameter) {
        this.parameter = parameter;
    }

    @Override
    public String toString() {
        return "Car{" +
                "id=" + id +
                ", brand='" + brand + '\'' +
                ", color='" + color + '\'' +
                ", parameter='" + parameter + '\'' +
                '}' + "\n";
    }
}

Dao类
MeetingDao类

package cn.zl.jdbc3.dao;

import cn.zl.jdbc3.entity.Boss;
import cn.zl.jdbc3.entity.Meeting;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.List;

public class MeetingDao {

    public Meeting findMeeting(int id)throws Exception{
        Class.forName("com.mysql.jdbc.Driver");
        Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/boss?useSSL=false",
                "root", "123456");
        PreparedStatement statement = connection.prepareStatement("select *from meeting where id=?");
        statement.setObject(1,id);
        ResultSet resultSet = statement.executeQuery();
        Meeting meeting=null;
        if (resultSet.next()){
            BossDao bossDao=new BossDao();
            int id1 = resultSet.getInt("id");
            List<Boss> list = bossDao.findBoss(id1);
            meeting=new Meeting();
            meeting.setId(id1);
            meeting.setTheme(resultSet.getString("theme"));
            meeting.setBossList(list);

        }
        resultSet.close();
        statement.close();
        connection.close();
        return meeting;
    }
}

BossDao类

package cn.zl.jdbc3.dao;

import cn.zl.jdbc3.entity.Boss;
import cn.zl.jdbc3.entity.Car;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.util.ArrayList;
import java.util.List;

public class BossDao {
    public List<Boss> findBoss(int id) throws Exception {
        Class.forName("com.mysql.jdbc.Driver");
        Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/boss?useSSL=false",
                "root", "123456");
        PreparedStatement statement = connection.prepareStatement("select *from boss where meeting_id=?");
        statement.setObject(1, id);
        ResultSet resultSet = statement.executeQuery();
        Boss boss = null;
        List<Boss> list = new ArrayList<>();
        while (resultSet.next()) {
            CarDao carDao = new CarDao();
            boss = new Boss();
            int id1 = resultSet.getInt("id");
            Car car = carDao.findCar(id1);
            boss.setId(id1);
            boss.setName(resultSet.getString("name"));
            boss.setCompany(resultSet.getString("company"));
            boss.setCar(car);
            list.add(boss);

        }
        return list;
    }
}

CarDao类

package cn.zl.jdbc3.dao;

import cn.zl.jdbc3.entity.Car;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

public class CarDao {
    public Car findCar(int id) throws Exception {
        Class.forName("com.mysql.jdbc.Driver");
        Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/boss?useSSL=false",
                "root", "123456");
        PreparedStatement statement = connection.prepareStatement("select *from car where id=?");
        statement.setObject(1, id);
        ResultSet resultSet = statement.executeQuery();
        Car car = null;
        if (resultSet.next()) {
            car = new Car();
            car.setId(resultSet.getInt("id"));
            car.setBrand(resultSet.getString("brand"));
            car.setColor(resultSet.getString("color"));
            car.setParameter(resultSet.getString("parameter"));
        }
        resultSet.close();
        statement.close();
        connection.close();

        return car;
    }
}

主类
DbTest01

public static void main(String[] args) throws Exception {
        MeetingDao meetingDao=new MeetingDao();
        Meeting meeting = meetingDao.findMeeting(1);
        System.out.println(meeting);
    }