言曌

Mybatis高级映射一对多查询

本文目标：以主表（orders订单表）和另外两个关联表（orderdetail 订单详细表，user 用户表）进行一对多查询。

一、开发准备

1、新建数据表（四张表）和添加测试数据

DROP TABLE IF EXISTS `items`;
DROP TABLE IF EXISTS `orders`;
DROP TABLE IF EXISTS `user`;
DROP TABLE IF EXISTS `orderdetail`;
/*items是商品表*/
CREATE TABLE `items` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(32) NOT NULL COMMENT '商品名称',
`price` FLOAT(10,1) NOT NULL COMMENT '商品定价',
`detail` TEXT COMMENT '商品描述',
`pic` VARCHAR(64) DEFAULT NULL COMMENT '商品图片',
`createtime` DATETIME NOT NULL COMMENT '生产日期',
PRIMARY KEY (`id`)
) ENGINE=INNODB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;
/*user是用户表*/
CREATE TABLE `user` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`username` VARCHAR(32) NOT NULL COMMENT '用户名称',
`birthday` DATE DEFAULT NULL COMMENT '生日',
`gender` CHAR(1) DEFAULT NULL COMMENT '性别',
`address` VARCHAR(256) DEFAULT NULL COMMENT '地址',
PRIMARY KEY (`id`)
) ENGINE=INNODB AUTO_INCREMENT=27 DEFAULT CHARSET=utf8;
/*orders是订单表*/
CREATE TABLE `orders` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`user_id` INT(11) NOT NULL COMMENT '下单用户id',
`number` VARCHAR(32) NOT NULL COMMENT '订单号',
`createtime` DATETIME NOT NULL COMMENT '创建订单时间',
`note` VARCHAR(100) DEFAULT NULL COMMENT '备注',
PRIMARY KEY (`id`),
KEY `FK_orders_1` (`user_id`),
CONSTRAINT `FK_orders_id` FOREIGN KEY (`user_id`) REFERENCES `user` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=INNODB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;
/*orderdetail是订单明细表*/
DROP TABLE IF EXISTS orderdetail;
CREATE TABLE `orderdetail` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`orders_id` INT(11) NOT NULL COMMENT '订单id',
`items_id` INT(11) NOT NULL COMMENT '商品id',
`items_num` INT(11) DEFAULT NULL COMMENT '商品购买数量',
PRIMARY KEY (`id`),
KEY `FK_orderdetail_1` (`orders_id`),
KEY `FK_orderdetail_2` (`items_id`),
CONSTRAINT `FK_orderdetail_1` FOREIGN KEY (`orders_id`) REFERENCES `orders` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `FK_orderdetail_2` FOREIGN KEY (`items_id`) REFERENCES `items` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=INNODB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

为了测试，我这里随便填了些数据

2、思路分析

订单项和订单明细是一对多的关系，所以本文主要来查询订单表，然后关联订单明细表，这样就有一对多的问题出来了。

　首先还是先写sql语句，在写sql语句的时候遵循两点：

查询的主表是哪个？订单表

查询的关联表是哪个？订单明细表

明确了主表和关联表，下面就可以写sql了，我们在上一节的sql基础上添加订单明细表的关联即可。

--查询订单关联查询用户及订单明细-->
<select id="findOrdersAndOrderDetailResultMap" resultMap="OrdersAndOrderDetailResultMap">
SELECT
orders.*,
user.username,
user.gender,
user.address,
orderdetail.id orderdetail_id,
orderdetail.items_id,
orderdetail.items_num,
orderdetail.orders_id
FROM
orders,
user,
orderdetail
WHERE orders.user_id=user.id AND orders.id = orderdetail.orders_id
select>

这样我们就查询出了订单表中的所有字段，user表和orderdetail表的部分字段，当然也可以查询所有字段，这个根据具体需求来定。看一下查询结果：

二、代码实现

1、新建三个持久类

① User.java

package com.liuyanzhao.mybatis.po;
import java.util.Date;
/**
* 用户的持久类
*/
public class User {
private int id; //编号
private String username; //用户名
private String gender; //性别
private Date birthday; //生日
private String address; //地址
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getGender() {
return gender;
}
public void setGender(String gender) {
this.gender = gender;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
}

② Orderdetail.java

package com.liuyanzhao.mybatis.po;
/**
* 订单明细的持久类
*/
public class Orderdetail {
private int id;
private int ordersId;
private int itemsId;
private int itemsNum;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getOrdersId() {
return ordersId;
}
public void setOrdersId(int ordersId) {
this.ordersId = ordersId;
}
public int getItemsId() {
return itemsId;
}
public void setItemsId(int itemsId) {
this.itemsId = itemsId;
}
public int getItemsNum() {
return itemsNum;
}
public void setItemsNum(int itemsNum) {
this.itemsNum = itemsNum;
}
}

③ Orders.java

package com.liuyanzhao.mybatis.po;
import java.util.Date;
import java.util.List;
/**
* 订单的持久类和扩展类
*/
public class Orders {
private int id;
private int userId;
private String number;
private Date createTime;
private String note;
//用户信息
private User user;
//订单明细
private List orderdetails;
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public List getOrderdetails() {
return orderdetails;
}
public void setOrderdetails(List orderdetails) {
this.orderdetails = orderdetails;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public int getUserId() {
return userId;
}
public void setUserId(int userId) {
this.userId = userId;
}
public String getNumber() {
return number;
}
public void setNumber(String number) {
this.number = number;
}
public Date getCreateTime() {
return createTime;
}
public void setCreateTime(Date createTime) {
this.createTime = createTime;
}
public String getNote() {
return note;
}
public void setNote(String note) {
this.note = note;
}
}

注意：我们的orders 表作为主表，orders类当然需要扩展其他关联表的属性

2、OrdersMapperCustom.java

package com.liuyanzhao.mybatis.mapper;
import com.liuyanzhao.mybatis.po.Orders;
import java.util.List;
/**
* 订单 mapper
*/
public interface OrdersMapperCustom {
//查询订单(关联用户)及订单明细
public List findOrdersAndOrderDetailResultMap() throws Exception;
}

3、OrdersMapperCustom.xml 映射文件

xml version="1.0" encoding="UTF-8" ?>
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.liuyanzhao.mybatis.mapper.OrdersMapperCustom">
<resultMap id="OrdersAndOrderDetailResultMap" type="com.liuyanzhao.mybatis.po.Orders">
<select id="findOrdersAndOrderDetailResultMap" resultMap="OrdersAndOrderDetailResultMap">
SELECT
orders.*,
user.username,
user.gender,
user.address,
orderdetail.id orderdetail_id,
orderdetail.items_id,
orderdetail.items_num,
orderdetail.orders_id
FROM
orders,
user,
orderdetail
WHERE orders.user_id=user.id AND orders.id = orderdetail.orders_id
select>
mapper>

注意：

的 id 是一个唯一标识符；type 相当于 resultType，表示查询的返回值类型。