Get latest comments from multiple tables - php

Question

I'm having trouble with getting the 3 latest comments from two different tables.
Here is my code, which works perfect with one inner join:
$query = mysql_query("SELECT COUNT(c.topic_id) AS ctid, COUNT(c.deck_id) AS dtid, f.id AS forumid, f.class AS forumclass, f.name AS forumname, f.url AS forumurl,
c.id AS commentid, c.user_id AS commentuser, c.user_name AS commentusername, c.date AS commentdate,
c.topic_id AS topicid, c.deck_id AS deckid
FROM ".$prefix."comment AS c
INNER JOIN ".$prefix."forum AS f
ON c.topic_id = f.id GROUP BY f.id
ORDER BY commentdate DESC LIMIT 3") or die(mysql_error());
This works fine it shows the 3 latest comments from the forum table, however I have comments in the decks table too, but when I add another JOIN to the query it just won't work anymore.
$query = mysql_query("SELECT COUNT(c.topic_id) AS ctid, COUNT(c.deck_id) AS dtid, f.id AS forumid, f.class AS forumclass, f.name AS forumname, f.url AS forumurl,
c.id AS commentid, c.user_id AS commentuser, c.user_name AS commentusername, c.date AS commentdate,
c.topic_id AS topicid, c.deck_id AS deckid, , d.id, d.url AS deckurl, d.name AS deckname
FROM ".$prefix."comment AS c
INNER JOIN ".$prefix."forum AS f
ON c.topic_id = f.id
INNER JOIN ".$prefix."decks AS d
ON c.deck_id = d.id
GROUP BY f.id ORDER BY commentdate DESC LIMIT 3") or die(mysql_error());
There's a comment table and in the comment table there is a topic_id column, which is equal to the forum table's id column and there's also a deck_id column which is equal to the decks table's id column.
Obviously that GROUP BY f.id isn't good with the two inner joins.
After this query I have while ($top = mysql_fetch_assoc($query)){ ... and then if ($top['deckid']==0) then print the topicid informations else print the deckid informations.
EDIT:
Comment table (only what is important to us now):
id, topic_id, deck_id
topic_id = forum table's id
deck_id = deck table's id
Obviously there is no comment to every forum topic.
E.g.: Forum ID 5 has 4 comments, then comment table e.g.: ID 1,2,3,4 has topic_id 4,4,4,4 and deck_id 0,0,0,0.
If there's no comment then there's nothing in the comment table. So forum ID 6 has 0 comments, then there is nothing in the comments table.
If deck ID 12 has 2 comments, then comment table e.g.: 5,6 has deck_id 12,12 and topic_id 0,0.
Forum table:
id
Deck table:
id
EDIT2: Solution (not too nice, but it works):
//count how many comments the latest 3 deck topic has
$new_comment_query = mysql_query("SELECT COUNT(c.deck_id) AS dtid, c.id, c.deck_id, c.date, d.id
FROM ".$prefix."comment AS c LEFT JOIN ".$prefix."decks AS d ON d.id = c.deck_id GROUP BY d.id ORDER BY date DESC LIMIT 3");
$new_one = mysql_fetch_array($new_comment_query);
//count how many comments the latest 3 forum topic has
$new_forum_query = mysql_query("SELECT COUNT(c.topic_id) AS ctid, c.id, c.topic_id, c.date, f.id
FROM ".$prefix."comment AS c LEFT JOIN ".$prefix."forum AS f ON f.id = c.topic_id GROUP BY f.id ORDER BY date DESC LIMIT 3");
$newer_one = mysql_fetch_array($new_forum_query);
//get all the comments
$comment_query = mysql_query("SELECT id, topic_id, deck_id, date FROM ".$prefix."comment ORDER BY date DESC LIMIT 3");
while ($comment = mysql_fetch_assoc($comment_query))
{
if($comment['topic_id']==0)
{
$deck_query = mysql_query("SELECT * FROM ".$prefix."decks WHERE id=".$comment['deck_id']);
while ($deck_comments = mysql_fetch_assoc($deck_query))
{
//print all the things!
}
}
elseif($comment['deck_id']==0)
{
$forum_query = mysql_query("SELECT * FROM ".$prefix."forum WHERE id=".$comment['topic_id']);
while ($forum_comments = mysql_fetch_assoc($forum_query))
{
//print all the things!
}
}
}

Answer 1

Try running this query:
SELECT * FROM
(
SELECT COUNT(c.topic_id) AS ctid, COUNT(c.deck_id) AS dtid,
f.id AS forumid, f.class AS forumclass, f.name AS forumname,
f.url AS forumurl,
c.id AS commentid, c.user_id AS commentuser, c.user_name AS commentusername,
c.date AS commentdate, c.topic_id AS topicid, c.deck_id AS deckid,
d.id, d.url AS deckurl, d.name AS deckname
FROM ".$prefix."comment AS c
INNER JOIN ".$prefix."forum AS f
ON c.topic_id = f.id
INNER JOIN ".$prefix."decks AS d
ON c.deck_id = d.id
ORDER BY commentdate DESC
) t1
GROUP BY t1.forumid
LIMIT 3
I first execute your complex JOIN without the GROUP BY which was causing problems. Then I SELECT everything from that temporary table, grouping by the forumid.
In all honesty, I'm surprised your server wasn't barfing on your first query, let alone the second, but hopefully my answer will help you out.

Answer 2

//count how many comments the latest 3 deck topic has
$new_comment_query = mysql_query("SELECT COUNT(c.deck_id) AS dtid, c.id, c.deck_id, c.date, d.id
FROM ".$prefix."comment AS c LEFT JOIN ".$prefix."decks AS d ON d.id = c.deck_id GROUP BY d.id ORDER BY date DESC LIMIT 3");
$new_one = mysql_fetch_array($new_comment_query);
//count how many comments the latest 3 forum topic has
$new_forum_query = mysql_query("SELECT COUNT(c.topic_id) AS ctid, c.id, c.topic_id, c.date, f.id
FROM ".$prefix."comment AS c LEFT JOIN ".$prefix."forum AS f ON f.id = c.topic_id GROUP BY f.id ORDER BY date DESC LIMIT 3");
$newer_one = mysql_fetch_array($new_forum_query);
//get all the comments
$comment_query = mysql_query("SELECT id, topic_id, deck_id, date FROM ".$prefix."comment ORDER BY date DESC LIMIT 3");
while ($comment = mysql_fetch_assoc($comment_query))
{
if($comment['topic_id']==0)
{
$deck_query = mysql_query("SELECT * FROM ".$prefix."decks WHERE id=".$comment['deck_id']);
while ($deck_comments = mysql_fetch_assoc($deck_query))
{
//print all the things!
}
}
elseif($comment['deck_id']==0)
{
$forum_query = mysql_query("SELECT * FROM ".$prefix."forum WHERE id=".$comment['topic_id']);
while ($forum_comments = mysql_fetch_assoc($forum_query))
{
//print all the things!
}
}
}