问题描述
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我有3张桌子的房间,酒店,城市,我该如何选择每个城市的十个最便宜的房间及其hotel_id
可以为每个城市执行子查询?
SELECT price,room_id,hotel_id,city_id
FROM Hotel
JOIN Room USING(hotel_id)
ORDER BY price
LIMIT 10
Scheme
CREATE TABLE `City` (
`city_id` int(11) NOT NULL AUTO_INCREMENT,`city_name` varchar(50) NOT NULL,PRIMARY KEY (`city_id`)
)
CREATE TABLE `Hotel` (
`hotel_id` int(11) NOT NULL AUTO_INCREMENT,`city_id` int(11) NOT NULL,`hotel_name` varchar(100) NOT NULL,PRIMARY KEY (`hotel_id`),CONSTRAINT `Hotel_FK_1` FOREIGN KEY (`city_id`) REFERENCES `City` (`city_id`) ON DELETE CASCADE ON UPDATE CASCADE
)
CREATE TABLE `Room` (
`room_id` int(11) NOT NULL AUTO_INCREMENT,`hotel_id` int(11) NOT NULL,`room_name` varchar(255) DEFAULT NULL,`room_price1` int(11) DEFAULT NULL,PRIMARY KEY (`room_id`),CONSTRAINT `Room_FK_1` FOREIGN KEY (`hotel_id`) REFERENCES `Hotel` (`hotel_id`) ON DELETE CASCADE ON UPDATE CASCADE
)
用子查询查询
SELECT
r.room_price1,r.room_id,h.hotel_name,c.city_name
FROM Hotel h
INNER JOIN Room r ON
h.hotel_id = r.hotel_id
INNER JOIN City c USING(city_id)
WHERE (
SELECT count(*) from Room as rt
JOIN Hotel ht using(hotel_id)
where c.city_id = ht.city_id and rt.room_price1 < r.room_price1 OR (rt.room_price1 = r.room_price1 AND rt.room_id < r.room_id)
) < 3
ORDER BY c.city_name,h.hotel_id,r.room_price1
如果某些酒店有一些room_price1,则此查询返回多于3行的城市信息,并且我需要每个城市3个唯一的酒店,此查询可以从一个酒店返回3个房间
请尝试使用用户变量
set @num := 0,@type := \'\';
select *
from (
select r.room_price1 pr,c.city_name,@num := if(@type = city_id,@num + 1,1) as row_number,@type := city_id as dummy
from Hotel h
JOIN Room r USING(hotel_id)
INNER JOIN City c USING(city_id)
order by city_name,r.room_price1 asc
) as x where x.row_number <=3;
对于row_number <6(我的初始化数据),此方法返回错误的排序
解决方法
那里有很多关于该主题的博客文章。
对我来说最有价值的可能是
Xaprb帖子(有关详细说明,请参见“从每个组中选择前N行”一节)
http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/
在您的情况下,这意味着。根据您的需求对该SQL进行建模。
select type,variety,price
from fruits
where (
select count(*) from fruits as f
where f.type = fruits.type and f.price < fruits.price
) <= 2;
让我们尝试一下
SELECT
r.price,r.room_id,h.hotel_id,h.city_id
FROM Hotel h INNER JOIN Room r ON
h.hotel_id = r.hotel_id
WHERE (
select count(*) from Room as rt
where r.hotel_id = rt.hotel_id and r.price > rt.price
) <= 10;
ORDER BY h.city_id,r.price
如您所见,您可以进行子查询以选择所有房间的数量
价格低于当前根的价格,看看是否
计数小于或等于10。
特别要注意我如何改变
f.price < fruits.price
至
r.price > rt.price
因为在Xaprb的示例中,他选择了N个最便宜的水果
希望能有所帮助。
BTW Stackoverflow并非用于解决作业。您也应该学习一些自己的东西;-)
,我假设表“ 7”引用了它所属的“ 8”
SELECT price,room_id,hotel_id,city_id
FROM Hotel,Room
WHERE Hotel.hotel_id = Room.hotel_id
ORDER BY price GROUP BY city_id
LIMIT 10