MySQL query - possible to include this clause? - sql

Question

I have the following query, which retrieves 4 adverts from certain categories in a random order.
At the moment, if a user has more than 1 advert, then potentially all of those ads might be retrieved - I need to limit it so that only 1 ad per user is displayed.
Is this possible to achieve in the same query?
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

Answer 1

Note: The solution is the last query at the bottom of this answer.
Test Schema and Data
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)
;
Data properties
each website can belong to multiple categories
for simplicity, all adverts are of type 'text'
each advert can belong to multiple categories. If a website has multiple categories that are matched multiple times in advert_categories for the same user_id, this causes the advert_id's to show twice when using a straight join between 3 tables in the next query.
This query joins the 3 tables together (notice that ids 1, 3 and 10 each appear twice)
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
To make each website show only once, this is the core query to show all eligible ads, each only once
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)
The next query retrieves all the advert_id's to be shown
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
There are 3 levels to this query
aliased EligibleAdsAndUserIDs: core query, sorted randomly using order by rand()
aliased RowNumbered: row number added to core query, using MySQL side-effecting #variables
the outermost query forces mysql to collect rows as numbered randomly in the inner queries, and group by user_id causes it to retain only the first row for each user_id. limit 2 causes the query to stop as soon as two distinct user_id's have been encountered.
This is the final query which takes the advert_id's from the previous query and joins it back to table adverts to retrieve the required columns.
only once per user_id
feature user's with more ads proportionally (statistically) to the number of eligible ads they have
Note: Point (2) works because the more ads you have, the more likely you will hit the top placings in the row numbering subquery
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;

Answer 2

I'm thinking through something but don't have MySQL available.. can you try this query to see if it works or crashes...
SELECT
PreQuery.user_id,
(select max( tmp.someRandom ) from PreQuery tmp where tmp.User_ID = PreQuery.User_ID ) MaxRandom
from
( select adverts.user_id,
rand() someRandom
from adverts, advert_categories
where adverts.advert_id = advert_categories.advert_id ) PreQuery
If the "tmp" alias is recognized as a temp buffer of the preliminary query as defined by the OUTER FROM clause, I might have something that will work... I think the field as a select statement from a queried from WONT work, but if it does, I know I'll have something solid for you.

Answer 3

Ok, this one might make the head hurt a bit, but lets get the logical thing going... The inner most "Core Query" is a basis that gets all unique and randomly assigned QUALIFIED Users that have a qualifying ad base on the category chosen, and type = 'text'. Since the order is random, I don't care what the assigned sequence is, and order by that. The limit 4 will return the first 4 entries that qualify. This is regardless of one user having 1 ad vs another having 1000 ads.
Next, join to the advertisements, reversing the table / join qualifications... but by having a WHERE - IN SUB-SELECT, the sub-select will be on each unique USER ID that was qualified by the "CoreQuery" and will ONLY be done 4 times based on ITs inner limit. So even if 100 users with different advertisements, we get 4 users.
Now, the Join to the CoreQuery is the Advert Table based on the same qualifying user. Typically this would join ALL records against the core query given they are for the same user in question... This is correct... HOWEVER, the NEXT WHERE clause is what filters it down to only ONE ad for the given person.
The Sub-Select is making sure its "Advert_ID" matches the one selected in the sub-select. The sub-select is based ONLY on the current "CoreQuery.user_ID" and gets ALL the qualifying category / ads for the user (wrong... we don't want ALL ads)... So, by adding an ORDER BY RAND() will randomize only this one person's ads in the result set... then Limiting THAT by 1 will only give ONE of their qualified ads...
So, the CoreQuery restricts down to 4 users. Then for each qualified user ID, gets only 1 of the qualified ads (by its inner order by RAND() and LIMIT 1 )...
Although I don't have MySQL to try, the queries are COMPLETELY legit and hope it works for you.... man, I love brain teasers like this...
SELECT
ad1.*
from
( SELECT ad.user_id,
count(*) as UserAdCount,
RAND() as ANYRand
from
website_categories wc
inner join advert_categories ac
ON wc.category_id = ac.category_id
inner join adverts ad
ON ac.advert_id = ad.advert_id
AND ad.advert_type = 'text'
where
wc.website_id = 8
GROUP BY
1
order by
3
limit
4 ) CoreQuery,
adverts ad1
WHERE
ad1.advert_type = 'text'
AND CoreQuery.User_ID = ad1.User_ID
AND ad1.advert_id in
( select
ad2.advert_id
FROM
adverts ad2,
advert_categories ac2,
website_categories wc2
WHERE
ad2.user_id = CoreQuery.user_id
AND ad2.advert_id = ac2.advert_id
AND ac2.category_id = wc2.category_id
AND wc2.website_id = 8
ORDER BY
RAND()
LIMIT
1 )

Answer 4

I like to suggest that you do the random with php. This is way faster than doing it in mySQL.
"However, when the table is large (over about 10,000 rows) this method of selecting a random row becomes increasingly slow with the size of the table and can create a great load on the server. I tested this on a table I was working that contained 2,394,968 rows. It took 717 seconds (12 minutes!) to return a random row."
http://www.greggdev.com/web/articles.php?id=6

Answer 5

set #userid = -1;
select
a.id,
a.title,
case when #userid = a.userid then
0
else
1
end as isfirst,
(#userid := a.userid)
from
adverts a
inner join advertcategories ac on ac.advertid = a.advertid
inner join categories c on c.categoryid = ac.categoryid
where
c.website = 8
order by
a.userid,
rand()
having
isfirst = 1
limit 4

Answer 6

Add COUNT(a.user_id) as owned in the main select directive and add HAVING owned < 2 after Group By
http://dev.mysql.com/doc/refman/5.5/en/select.html
I think this is the way to do it, if the one user has more than one advert then we will not select it.