多表查询

    初始化数据:
    -- 用户表(user) 
       create table `user` (                                  
              `id` int auto_increment primary key,                
              `username` varchar(50)  -- 用户姓名                                                
            );

    -- 订单表(orders)
       create table `orders` (                                                  
              `id` int  auto_increment primary key,                                  
              `price` double,                                           
              `user_id` int                                       
            );
    -- 给订单表添加外键约束
    alter table orders add constraint user_fk foreign key (user_id) references user(id); 

    -- 向user表中添加数据
            insert into user values(3,‘张三‘);
            insert into user values(4,‘李四‘);
            insert into user values(5,‘王五‘);
            insert into user values(6,‘赵六‘);

    -- 向orders 表中插入数据
            insert into orders values(1,1314,3);
            insert into orders values(2,1314,3);
            insert into orders values(3,15,4);
            insert into orders values(4,315,5);
            insert into orders values(5,1014,null);