mysql - subqueries and joins - php

Question

I'm not quite sure if this is the right approach, this is my situation:
I'm currently trying to select 15 galleries and then left join it with the user table through the id but I also want to select one random picture from each gallery however from what I know you can't limit the left join (picture) to only pick up one random picture without doing a subquery.
Here is what I got so far but its not working as it should:
SELECT galleries.id, galleries.name, users.username, pictures.url
FROM galleries
LEFT JOIN users ON users.id = galleries.user_id
LEFT JOIN pictures ON (
SELECT pictures.url
FROM pictures
WHERE pictures.gallery_id = galleries.id
ORDER BY RAND()
LIMIT 1)
WHERE active = 1
ORDER BY RAND()
LIMIT 15
I also tried to do this with Active Record but I got stuck after doing two left joins, is it possible to do get a subquery in here:
$this->db->select('galleries.id, galleries.name, users.id as user_id, users.username');
$this->db->from('galleries');
$this->db->join('users', 'users.id = galleries.user_id','left');
$this->db->join('pictures','pictures.gallery_id = galleries.id AND','left');
$this->db->where('active',1);
I hope its not to messy but I'm really starting to get confusing by all the sql queries..
Edit:
Active Record with CodeIgniter

Answer 1

You could fetch a random picture in a subquery:
select
g.name, u.username,
(select url from pictures p where p.gallery_id = g.gallery_id
order by rand() limit 1) as url
from galleries g
left join users u on g.user_id = u.id
where g.active = 1
Based on your comment, you could select a picture for each gallery in a subquery. This is assuming the picture table has an ID column.
select
g.name, u.username, p.url, p.name
from (
select id, user_id, name,
(select id from pictures p
where p.gallery_id = g.gallery_id
order by rand() limit 1) as samplepictureid
from galleries
where g.active = 1
) g
left join users u on g.user_id = u.id
left join pictures p on p.id = g.samplepictureid

Answer 2

SELECT
g.id,
g.name,
u.username,
p.url
FROM
galleries g
INNER JOIN (SELECT DISTINCT
gallery_id,
(SELECT url FROM pictures ss WHERE ss.gallery_id = s.gallery_id
ORDER BY RAND() LIMIT 1) AS url
FROM
pictures s) p ON
g.id = p.gallery_id
LEFT OUTER JOIN users u ON
g.user_id = u.id
WHERE
g.active = 1
This query will go out and select a gallery, then it will find any gallery with a picture (if you want to return galleries without a picture, change INNER JOIN to LEFT OUTER JOIN, and you'll be fine). After that, it joins it up with users. Now, of course, this puppy is going to return every frigging gallery for however many users you have (hoorah!). You may want to limit the user in the WHERE clause (e.g.-WHERE u.id = 123). Otherwise, you're going to get more results than you'd expect. That, or do an INNER JOIN on it.