Question

我无法从两个不同的表格中获取3条最新评论。

这是我的代码，它与一个内连接完美配合：

$query = mysql_query("SELECT COUNT(c.topic_id) AS ctid, COUNT(c.deck_id) AS dtid, f.id AS forumid, f.class AS forumclass, f.name AS forumname, f.url AS forumurl,
            c.id AS commentid, c.user_id AS commentuser, c.user_name AS commentusername, c.date AS commentdate,
            c.topic_id AS topicid, c.deck_id AS deckid
            FROM ".$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());

这很好，它显示了论坛表中的3条最新评论，但是我也在甲板表中有评论，但是当我在查询中添加另一个JOIN时，它就不再工作了。

$query = mysql_query("SELECT COUNT(c.topic_id) AS ctid, COUNT(c.deck_id) AS dtid, f.id AS forumid, f.class AS forumclass, f.name AS forumname, f.url AS forumurl,
            c.id AS commentid, c.user_id AS commentuser, c.user_name AS commentusername, c.date AS commentdate,
            c.topic_id AS topicid, c.deck_id AS deckid, , d.id, d.url AS deckurl, d.name AS deckname
            FROM ".$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());

有一个评论表，在评论表中有一个topic_id列，它等于论坛表的id列，还有一个deck_id列，它等于甲板table的id列。

显然GROUP BY f.id对两个内部联接并不好。

在此查询之后我有了（$ top = mysql_fetch_assoc（$ query））{...然后if（$ top [＆＃39; deckid＆＃39;] == 0）然后打印topicid信息else print甲板上的信息。

编辑：评论表（现在只对我们很重要）： id，topic_id，deck_id

topic_id =论坛表格的ID

deck_id = deck table＆＃39; id

显然，每个论坛主题都没有评论。

例如：论坛ID 5有4条评论，然后是评论表，例如：ID 1,2,3,4有topic_id 4,4,4,4和deck_id 0,0,0,0。

如果没有评论，则评论表中没有任何内容。所以论坛ID 6有0条评论，然后评论表中没有任何内容。

如果牌组12有2条评论，那么评论表例如：5,6有deck_id 12,12和topic_id 0,0。

论坛表： ID

甲板表： ID

EDIT2：解决方案（不太好，但有效）：

//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

尝试运行此查询：

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

我首先执行您的复杂JOIN 而不导致问题的GROUP BY。然后我SELECT来自该临时表的所有内容，按forumid进行分组。

老实说，我很惊讶你的服务器没有对你的第一个查询进行抨击，更不用说第二个问题，但希望我的回答可以帮助你。

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!
            }
        }
    }

从多个表

2 个答案: