How to set limit for joined mysql table? - node.js

Question

I'm trying to combine data from 3 tables with below query
SELECT `media_category`.id as cat_id,
`media_category`.category_name as cat_name,
`video`.`id` as vid_id,
`video`.`name` as vid_name,
`screenshots`.name as screenshot_name
FROM `media_category`
LEFT JOIN video
ON `media_category`.id = `video`.`category_id`
LEFT JOIN screenshots
ON `video`.id = `screenshots`.`media_id`
WHERE `screenshots`.name NOT LIKE '%\_%'
version: mysql 5.7
It workes well. But I need to limit the rows getting from video table to LIMIT 10 per category
Any idea for that?

Answer 1

You don't mention which MySQL version you are using so I'll assume it's MySQL 8.x.
In MySQL 8.x you can use the DENSE_RANK() function to identify the rows you want. Then a simple predicate will remove the ones you don't want.
For example, if we want to limit to 2 rows of b on each a (irrespective of rows in c), you can do:
select *
from (
select
a.id,
b.id as bid,
dense_rank() over(partition by a.id order by b.id) as drank,
c.id as cid
from a
left join b on b.aid = a.id
left join c on c.bid = b.id
) x
where drank <= 2
Result:
id bid drank cid
-- --- ----- ------
1 11 1 100
1 11 1 101
1 11 1 102
1 11 1 103
1 12 2 120
2 20 1 200
2 20 1 201
2 21 2 202
3 30 1 <null>
As you can see it shows only 11 and 12 for the id = 1, even though there are 5 total rows for it (all five are of rank 1 and 2). You can see the running example at DB Fiddle. The data script for this example is:
create table a (id int primary key not null);
insert into a (id) values (1), (2), (3);
create table b (id int primary key not null, aid int references a (id));
insert into b (id, aid) values
(11, 1), (12, 1), (13, 1), (14, 1),
(20, 2), (21, 2),
(30, 3);
create table c (id int primary key not null, bid int references b (id));
insert into c (id, bid) values
(100, 11), (101, 11), (102, 11), (103, 11),
(120, 12), (130, 13), (140, 14),
(200, 20), (201, 20),
(202, 21);