Left Join Subselect with LIMIT in MySQL - mysql

Question

I have 3 tables:
actor
| FIELD | TYPE | NULL | KEY | DEFAULT | EXTRA |
|----------|------------------|------|-----|---------|----------------|
| actor_id | int(10) unsigned | NO | PRI | (null) | auto_increment |
| username | varchar(30) | NO | | (null) | |
tag
| FIELD | TYPE | NULL | KEY | DEFAULT | EXTRA |
|--------|------------------|------|-----|---------|----------------|
| tag_id | int(10) unsigned | NO | PRI | (null) | auto_increment |
| title | varchar(40) | NO | | (null) | |
actor_tag_count
| FIELD | TYPE | NULL | KEY | DEFAULT | EXTRA |
|------------------|------------------|------|-----|-------------------|-----------------------------|
| actor_id | int(10) unsigned | NO | PRI | (null) | |
| tag_id | int(10) unsigned | NO | PRI | (null) | |
| clip_count | int(10) unsigned | NO | | (null) | |
| update_timestamp | timestamp | NO | | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
SQLFiddle
I want to get the 5 most frequent (highest clip_count) and most recently updated (latest update_timestamp) tags for each actor.
My attempted query is:
SELECT
`a`.`actor_id`,
`a`.`username`,
GROUP_CONCAT(atc.clip_count) AS `tag_clip_counts`,
GROUP_CONCAT(t.tag_id) AS `tag_ids`,
GROUP_CONCAT(t.title) AS `tag_titles`
FROM
`actor` AS `a`
LEFT JOIN (
SELECT
`atc`.`actor_id`,
`atc`.`tag_id`,
`atc`.`clip_count`
FROM
`actor_tag_count` AS `atc`
INNER JOIN `actor` AS `a` USING (actor_id)
ORDER BY
atc.clip_count DESC,
atc.update_timestamp DESC
LIMIT 5
) AS `atc` USING (actor_id)
LEFT JOIN `tag` AS `t` ON atc.tag_id = t.tag_id
GROUP BY
`a`.`actor_id`
The problem is that the left join subselect is only calculated once and the tags for every result in the set are only fetched from a pool of 5 tags.
Expected GROUP_CONCAT'd tag title results for Keanu Reeves:
comedy, scifi, action, suspense, western
(Both western and documentary have a clip_count of 2, but western should come first because it has a later update_timestamp)
I'm not sure this is a point of any relevance, but I am executing other joins on the actors table but had them removed for this question.
It would be highly preferable to make this all 1 query, but I'm stumped on how to do this even with 2 queries. 1-or-2-query solutions appreciated.

Answer 1

SQLFiddle, with the help of a very nice answer about using a GROUP_CONCAT limit workaround:
SELECT
`a`.`actor_id`,
`a`.`username`,
SUBSTRING_INDEX(GROUP_CONCAT(atc.clip_count ORDER BY atc.clip_count DESC, atc.update_timestamp DESC), ',', 5) AS `tag_clip_counts`,
SUBSTRING_INDEX(GROUP_CONCAT(t.tag_id ORDER BY atc.clip_count DESC, atc.update_timestamp DESC), ',', 5) AS `tag_ids`,
SUBSTRING_INDEX(GROUP_CONCAT(t.title ORDER BY atc.clip_count DESC, atc.update_timestamp DESC), ',', 5) AS `tag_titles`
FROM
`actor` AS `a`
LEFT JOIN actor_tag_count AS `atc` USING (actor_id)
LEFT JOIN `tag` AS `t` ON atc.tag_id = t.tag_id
GROUP BY
`a`.`actor_id`

Answer 2

It is possible by adding a sequence number, but might not perform well on large tables.
Something like this (not tested):-
SELECT actor_id,
username,
GROUP_CONCAT(clip_count) AS tag_clip_counts,
GROUP_CONCAT(tag_id) AS tag_ids,
GROUP_CONCAT(title) AS tag_titles
FROM
(
SELECT actor.actor_id,
actor.username,
atc.clip_count,
tag.tag_id,
tag.title,
#aSeq := IF(#aActorId = actor.actor_id, #aSeq, 0) + a AS aSequence,
#aActorId := actor.actor_id
FROM
(
SELECT actor.actor_id,
actor.username,
atc.clip_count,
tag.tag_id,
tag.title
FROM actor
LEFT JOIN actor_tag_count AS atc ON actor.actor_id = atc.actor_id
LEFT JOIN tag ON atc.tag_id = tag.tag_id
ORDER BY actor.actor_id, atc.clip_count DESC, atc.update_timestamp DESC
)
CROSS JOIN (SELECT #aSeq:=0, #aActorId:=0)
)
WHERE aSequence <= 5
GROUP BY actor_id, username
A alternative would be to have a subselect that has a correlated sub query in the select statement (with a limit of 5), and then have an outer query that does the group concats. Something like this (again not tested)
SELECT
actor_id,
username,
GROUP_CONCAT(clip_count) AS tag_clip_counts,
GROUP_CONCAT(tag_id) AS tag_ids,
GROUP_CONCAT(title) AS tag_titles
FROM
(
SELECT
a.actor_id,
a.username,
(
SELECT
atc.clip_count,
t.tag_id,
t.title
FROM actor_tag_count AS atc ON a.actor_id = atc.actor_id
LEFT JOIN tag t ON atc.tag_id = t.tag_id
ORDER BY atc.clip_count DESC, atc.update_timestamp DESC
LIMIT 5
)
FROM actor a
)
GROUP BY actor_id, username