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

1. 别名 EligibleAdsAndUserIDs:核心查询，使用 order by rand() 随机排序
2. 别名 RowNumbered:添加到核心查询的行号，使用 MySQL side-effecting @variables
3. 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。

1. 每个 user_id 一次
2. 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/