Question

$threads = mysql_query("SELECT DISTINCT t.id, t.title
                        FROM threads t LEFT JOIN comments c ON t.id = c.parentID
                        ORDER BY c.date DESC");

while ($thread = mysql_fetch_assoc($threads)) {

 echo "<li><a href=\"?threadID=$thread[id]\">".htmlspecialchars($thread['title'])."</a></li>\n";

}

Can anyone see a problem in this code? It wont work as i want it. I want it the thread that was commented in latest on top, and the the next latest etc.

echo $thread[parentID] echoes nothing

comments:
id, comment, parentID, date

Answer 1

You are ordering by a column not in your DISTINCT list.

This is valid syntax in MySQL, however this ORDER BY is meaningless.

This syntax is a MySQL extension, this query will fail in any other RDBMS engine of the big four.

It is used to simplify the DISTINCT and GROUP BY queries in the cases when same value of column in SELECT or ORDER BY always corresponds to same value of column in GROUP BY or DISTINCT.

If your case, you can have multiple values of c.date for each value of t.id, and which value of c.date will be selected for ORDER BY is far from being guaranteed (it can be the last value, the first value or any other value).

Rewrite your query as this:

SELECT  t.id, t.title,
        (
        SELECT  c.date
        FROM    comments c
        WHERE   c.parent_id = t.id
        ORDER BY
                c.date DESC
        LIMIT 1
        ) lastcomment
FROM    threads t
ORDER BY
        ISNULL(lastcomment), lastcomment