MyBatis多对多关联查询
例如，一个订单可以有多种商品，一种商品可以对应多个订单，订单与商品就是多对多的级联关系。可以使用一个中间表（订单记录表）将多对多级联转换成两个一对多的关系。
示例
下面以订单和商品（实现“查询所有订单以及每个订单对应的商品信息”的功能）为例讲解多对多关联查询。1. 创建数据表
创建 order（订单），product（商品）和 order_details（订单和商品中间表），SQL 语句如下。
CREATE TABLE `order` (
`oid` int(11) NOT NULL AUTO_INCREMENT,
`ordernum` int(25) DEFAULT NULL,
`userId` int(11) DEFAULT NULL,
PRIMARY KEY (`oid`),
KEY `userId` (`userId`),
CONSTRAINT `order_ibfk_1` FOREIGN KEY (`userId`) REFERENCES `user` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=10 DEFAULT CHARSET=utf8;
insert into `order`(`oid`,`ordernum`,`userId`) values (1,20200107,1),(2,20200806,2),(3,20206702,3),(4,20200645,1),(5,20200711,2),(6,20200811,2),(7,20201422,3),(8,20201688,4),(9,NULL,5);
DROP TABLE IF EXISTS `orders_detail`;
CREATE TABLE `orders_detail` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`orderId` int(11) DEFAULT NULL,
`productId` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;
insert into `orders_detail`(`id`,`orderId`,`productId`) values (1,1,1),(2,1,2),(3,1,3),(4,2,3),(5,2,1),(6,3,2);
DROP TABLE IF EXISTS `product`;
CREATE TABLE `product` (
`pid` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(25) DEFAULT NULL,
`price` double DEFAULT NULL,
PRIMARY KEY (`pid`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
insert into `product`(`pid`,`name`,`price`) values (1,'Java教程',128),(2,'C语言教程',138),(3,'Python教程',132.35);
2. 创建持久化类
Order 类代码如下。
package net.biancheng.po;
import java.util.List;
public class Order {
private int oid;
private int ordernum;
private List<Product> products;
/*省略setter和getter方法*/
@Override
public String toString() {
return "Order [id=" + oid + ", ordernum=" + ordernum + ", products=" + products + "]";
}
}
Product 类方法如下。
package net.biancheng.po;
import java.util.List;
public class Product {
private int pid;
private String name;
private Double price;
// 多对多中的一个一对多
private List<Order> orders;
/*省略setter和getter方法*/
@Override
public String toString() {
return "Product [id=" + pid + ", name=" + name + ", price=" + price + "]";
}
}
3. 创建接口和映射文件
OrderMapper 接口代码如下。
package net.biancheng.mapper;
import java.util.List;
import net.biancheng.po.Order;
public interface OrderMapper {
public List<Order> selectAllOrdersAndProducts();
}
OrderMapper.xml 代码如下。
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis网站站点" rel="nofollow" />
package net.biancheng.test;
import java.io.IOException;
import java.io.InputStream;
import java.util.List;
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 net.biancheng.mapper.OrderMapper;
import net.biancheng.po.Order;
public class Test {
public static void main(String[] args) throws IOException {
InputStream config = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactory ssf = new SqlSessionFactoryBuilder().build(config);
SqlSession ss = ssf.openSession();
List<Order> orderList = ss.getMapper(OrderMapper.class).selectAllOrdersAndProducts();
for (Order or : orderList) {
System.out.println(or);
}
}
}
运行结果如下。
DEBUG [main] - ==> Preparing: SELECT o.oid,o.`ordernum`,p.`pid`,p.`name`,p.`price` FROM `order` o INNER JOIN orders_detail od ON o.oid=od.`orderId` INNER JOIN product p ON p.pid = od.`productId`
DEBUG [main] - ==> Parameters:
DEBUG [main] - <== Total: 6
Order [id=1, ordernum=20200107, products=[Product [id=1, name=Java教程, price=128.0], Product [id=2, name=C语言教程, price=138.0], Product [id=3, name=Python教程, price=132.35]]]
Order [id=2, ordernum=20200806, products=[Product [id=3, name=Python教程, price=132.35], Product [id=1, name=Java教程, price=128.0]]]
Order [id=3, ordernum=20206702, products=[Product [id=2, name=C语言教程, price=138.0]]]