MYSQL how to limit results from join - mysql

Question

I have a query like this:
SELECT
p.title,
a.title
FROM
pages p
LEFT JOIN
articles_pages ap on ap.p_id = p.id
LEFT JOIN
articles a ON a.id = p.a_id
WHERE
[...]
LIMIT 10
How can I limit to only 3 articles for each page?

Answer 1

There are few ways to do that. You can use sub-queries but I wouldn't recommend that as its slower and not scalable. So.. I like the one I show you bellow:
I would create a variable and save on it the row number(#num).
We also need another variable to save the "page" id as its necessary to restart the row count when a different page appears.
Finally we filter by #num with as much rows you want. Remember matchNO is a calculated field so you cant filter in a where statment, use having instead.
set #num := 0, #page := 0;
SELECT p.title, a.title,IF(#page = p.id,#num:=#num+1,#num:=1) as matchNO,#page:=p.id
FROM pages p
LEFT JOIN articles_pages ap on ap.p_id = p.id
LEFT JOIN articles a ON a.id = p.a_id
WHERE [...]
having matchNO <= 3
LIMIT 10
The row number is necessary to figure out in what row you are.
The page variable tell you the page in the last row so you can compare and then set #num to 1 again when necesary.
I hope it helps

Answer 2

Another possible way to do it. Whether this would be better or worse than the others would depend on the relative quantities of pages and articles.
You could use a sub query to get 3 articles per page and then join that back to your tables. You can cheat a bit and use GROUP_CONCAT to get the ids of all the articles per per, then use SUBSTRING_INDEX to get the first 3 (you can add an order clause to GROUP_CONCAT if you want), and then JOIN this using FIND_IN SET.
SELECT
p.title,
a.title
FROM pages p
LEFT OUTER JOIN
(
SELECT ap.p_id
SUBSTRING_INDEX(GROUP_CONCAT(ap.a_id), ',', 3) AS three_articles_per_page
FROM articles_pages ap
GROUP BY ap.p_id
) sub0 ON p.id = sub0.p_id
LEFT JOIN articles_pages ap on ap.p_id = p.id AND FIND_IN_SET(ap.aid, sub0.three_articles_per_page)
LEFT JOIN articles a ON a.id = p.a_id
WHERE
[...]

Answer 3

You can make use of user variables to generate row number and then filter based on it.
select p.title, a.title
from (
select
t.*,
#rn := if(
#id = id, #rn + 1,
if(
#id := id, 1, 1
)
) rn
from (
select *
from pages
order by id, a_id
) t cross join (select #rn := 0, #id:= -1) t2
) p left join articles a on a.id = p.a_id
where p.rn <= 3
order by p.id, p.a_id;
Demo