How do I get a DISTINCT in this query - mysql

Question

this is my query:
SELECT phpbb_attachments.attach_id, phpbb_attachments.post_msg_id
FROM phpbb_attachments
INNER JOIN phpbb_topics
ON phpbb_attachments.topic_id = phpbb_topics.topic_id
INNER JOIN phpbb_forums ON phpbb_topics.forum_id = phpbb_forums.forum_id
WHERE phpbb_attachments.is_orphan = 0
AND phpbb_attachments.thumbnail = 1
AND (phpbb_attachments.extension = 'jpg'
OR phpbb_attachments.extension = 'png'
OR phpbb_attachments.extension = 'gif')
AND phpbb_forums.forum_id BETWEEN 668 AND 679
ORDER BY phpbb_attachments.attach_id DESC
LIMIT 50
I want to get only the latest 1 attachment per topic.
So sort of like a distinct on phpbb_attachments.topic_id

Answer 1

This query gives the "best" attachment per ropic:
SELECT MAX(a.attach_id), t.topic_id
FROM phpbb_attachments a
INNER JOIN phpbb_topics t ON a.topic_id = t.topic_id
INNER JOIN phpbb_forums f ON t.forum_id = f.forum_id
WHERE a.is_orphan = 0
AND a.thumbnail = 1
AND (a.extension = 'jpg' OR a.extension = 'png' OR a.extension = 'gif')
AND f.forum_id BETWEEN 668 AND 679
GROUP BY t.topic_id
Then select attachments from that list:
SELECT x.attach_id, x.post_msg_id
FROM phpbb_attachments x INNER JOIN (SELECT MAX(a.attach_id) AS attach_id, t.topic_id
FROM phpbb_attachments a
INNER JOIN phpbb_topics t ON a.topic_id = t.topic_id
INNER JOIN phpbb_forums f ON t.forum_id = f.forum_id
WHERE a.is_orphan = 0
AND a.thumbnail = 1
AND (a.extension = 'jpg' OR a.extension = 'png' OR a.extension = 'gif')
AND f.forum_id BETWEEN 668 AND 679
GROUP BY t.topic_id) i ON x.attach_id = i.attach_id
ORDER BY x.attach_id DESC
LIMIT 50

Answer 2

The id of SELECT phpbb_attachments.attach_id, phpbb_attachments.post_msg_id is not the same.
Now when you execute that Script even using SELECT distinct phpbb_attachments.attach_id, phpbb_attachments.post_msg_id, it will not generate a distinctphpbb_attachments.attach_id`.
The Reason is:
It will creating two distinct with this fields phpbb_attachments.attach_id, phpbb_attachments.post_msg_id
Now if you need a Distinct to this phpbb_attachments.attach_id
You have to do it first distinct:
SELECT Distinct phpbb_attachments.attach_id
FROM phpbb_attachments
INNER JOIN phpbb_topics
ON phpbb_attachments.topic_id = phpbb_topics.topic_id
INNER JOIN phpbb_forums ON phpbb_topics.forum_id = phpbb_forums.forum_id
WHERE phpbb_attachments.is_orphan = 0
AND phpbb_attachments.thumbnail = 1
AND (phpbb_attachments.extension = 'jpg'
OR phpbb_attachments.extension = 'png'
OR phpbb_attachments.extension = 'gif')
AND phpbb_forums.forum_id BETWEEN 668 AND 679
ORDER BY phpbb_attachments.attach_id DESC
LIMIT 50
then use left join to that particular Distinct Data using phpbb_attachments.attach_id itself something like:
SELECT attach_id,post_msg_id
FROM
(SELECT Distinct phpbb_attachments.attach_id,
FROM phpbb_attachments
INNER JOIN phpbb_topics
ON phpbb_attachments.topic_id = phpbb_topics.topic_id
INNER JOIN phpbb_forums ON phpbb_topics.forum_id = phpbb_forums.forum_id
WHERE phpbb_attachments.is_orphan = 0
AND phpbb_attachments.thumbnail = 1
AND (phpbb_attachments.extension = 'jpg'
OR phpbb_attachments.extension = 'png'
OR phpbb_attachments.extension = 'gif')
AND phpbb_forums.forum_id BETWEEN 668 AND 679
ORDER BY phpbb_attachments.attach_id DESC
LIMIT 50) as firstDistinct
LEFT JOIN
(SELECT phpbb_attachments.attach_id, phpbb_attachments.post_msg_id
FROM phpbb_attachments
INNER JOIN phpbb_topics
ON phpbb_attachments.topic_id = phpbb_topics.topic_id
INNER JOIN phpbb_forums ON phpbb_topics.forum_id = phpbb_forums.forum_id
WHERE phpbb_attachments.is_orphan = 0
AND phpbb_attachments.thumbnail = 1
AND (phpbb_attachments.extension = 'jpg'
OR phpbb_attachments.extension = 'png'
OR phpbb_attachments.extension = 'gif')
AND phpbb_forums.forum_id BETWEEN 668 AND 679
ORDER BY phpbb_attachments.attach_id DESC
LIMIT 50) as secondNotDistinct
On firstDistinct.attach_id = secondNotDistinct.attach_id