我有以下查询,它以随机顺序从某些类别中检索 4 个广告。
目前,如果用户有超过 1 个广告,则可能会检索所有这些广告 - 我需要对其进行限制,以便每个用户只显示 1 个广告。
这可以在同一个查询中实现吗?
SELECT a.advert_id, a.title, a.url, a.user_id,
FLOOR(1 + RAND() * x.m_id) 'rand_ind'
FROM adverts AS a
INNER JOIN advert_categories AS ac
ON a.advert_id = ac.advert_id,
(
SELECT MAX(t.advert_id) - 1 'm_id'
FROM adverts t
) x
WHERE ac.category_id IN
(
SELECT category_id
FROM website_categories
WHERE website_id = '8'
)
AND a.advert_type = 'text'
GROUP BY a.advert_id
ORDER BY rand_ind
LIMIT 4
最佳答案
注意:解决方案是这个答案底部的最后一个查询。
测试架构和数据
create table adverts (
advert_id int primary key, title varchar(20), url varchar(20), user_id int, advert_type varchar(10))
;
create table advert_categories (
advert_id int, category_id int, primary key(category_id, advert_id))
;
create table website_categories (
website_id int, category_id int, primary key(website_id, category_id))
;
insert website_categories values
(8,1),(8,3),(8,5),
(1,1),(2,3),(4,5)
;
insert adverts (advert_id, title, user_id) values
(1, 'StackExchange', 1),
(2, 'StackOverflow', 1),
(3, 'SuperUser', 1),
(4, 'ServerFault', 1),
(5, 'Programming', 1),
(6, 'C#', 2),
(7, 'Java', 2),
(8, 'Python', 2),
(9, 'Perl', 2),
(10, 'Google', 3)
;
update adverts set advert_type = 'text'
;
insert advert_categories values
(1,1),(1,3),
(2,3),(2,4),
(3,1),(3,2),(3,3),(3,4),
(4,1),
(5,4),
(6,1),(6,4),
(7,2),
(8,1),
(9,3),
(10,3),(10,5)
;
数据属性
- 每个网站可以属于多个类别
- 为简单起见,所有广告均为“文本”类型
- 每个广告可以属于多个类别。如果网站有多个类别在同一 user_id 的 advert_categories 中多次匹配,这会导致在下一个查询中使用 3 个表之间的直接连接时 advert_id 显示两次。
此查询将 3 个表连接在一起(注意 ID 1、3 和 10 各出现两次)
select *
from website_categories wc
inner join advert_categories ac on wc.category_id = ac.category_id
inner join adverts a on a.advert_id = ac.advert_id and a.advert_type = 'text'
where wc.website_id='8'
order by a.advert_id
为了让每个网站只显示一次,这是显示所有符合条件的广告的核心查询,每个广告只显示一次
select *
from adverts a
where a.advert_type = 'text'
and exists (
select *
from website_categories wc
inner join advert_categories ac on wc.category_id = ac.category_id
where wc.website_id='8'
and a.advert_id = ac.advert_id)
下一个查询检索所有要显示的 advert_id
select advert_id, user_id
from (
select
advert_id, user_id,
@r := @r + 1 r
from (select @r:=0) r
cross join
(
# core query -- vvv
select a.advert_id, a.user_id
from adverts a
where a.advert_type = 'text'
and exists (
select *
from website_categories wc
inner join advert_categories ac on wc.category_id = ac.category_id
where wc.website_id='8'
and a.advert_id = ac.advert_id)
# core query -- ^^^
order by rand()
) EligibleAdsAndUserIDs
) RowNumbered
group by user_id
order by r
limit 2
这个查询有3个级别
- 别名
EligibleAdsAndUserIDs
:核心查询,使用order by rand()
随机排序 - 别名
RowNumbered
:添加到核心查询的行号,使用 MySQL side-effecting @variables - 最外层的查询强制 mysql 在内部查询中随机收集
as numbered
行,group by user_id
使其只保留每个 user_id 的第一行。limit 2
导致查询在遇到两个不同的 user_id 时立即停止。
这是最后一个查询,它从之前的查询中获取 advert_id,并将其连接回表 adverts
以检索所需的列。
- 每个 user_id 一次
- 根据用户拥有的符合条件的广告数量,按比例(统计上)向用户展示更多广告
注意:第 (2) 点之所以有效,是因为您拥有的广告越多,您就越有可能在行编号子查询中排名靠前
select a.advert_id, a.title, a.url, a.user_id
from
(
select advert_id
from (
select
advert_id, user_id,
@r := @r + 1 r
from (select @r:=0) r
cross join
(
# core query -- vvv
select a.advert_id, a.user_id
from adverts a
where a.advert_type = 'text'
and exists (
select *
from website_categories wc
inner join advert_categories ac on wc.category_id = ac.category_id
where wc.website_id='8'
and a.advert_id = ac.advert_id)
# core query -- ^^^
order by rand()
) EligibleAdsAndUserIDs
) RowNumbered
group by user_id
order by r
limit 2
) Top2
inner join adverts a on a.advert_id = Top2.advert_id;
关于sql - MySQL 查询 - 可以包含这个子句吗?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/4699926/