如何从2个单独的SQL表中选择最新日期？

Question

Original problem

I have a code which with a while loop fetches a bunch of stuff, I ran against the problem that the while loop does not order correctly.

In this case it only sorts on which thread was posted latest, what I want is that it checks whether the latest thing in the database was a thread or a post.

$handler is my database connection variable, I use PDO.

My current code:

<?php
    $query = $handler->query('SELECT * FROM thread ORDER BY postdate DESC');

    while($fetch = $query->fetch(PDO::FETCH_ASSOC)){
        $rcount = $handler->query('SELECT COUNT(*) FROM threadpost WHERE t_id = ' . $fetch['t_id']);
        $queryTime = $handler->query('SELECT * FROM threadpost WHERE t_id =' . $fetch['t_id'] . ' ORDER BY postdate DESC');

        $fetchTime = $queryTime->fetch(PDO::FETCH_ASSOC);
        $rfetch = $rcount->fetch(PDO::FETCH_NUM);

        if(strtotime($fetch['postdate']) < strtotime($fetchTime['postdate'])){
            $fetch['postdate'] = $fetchTime['postdate'];
            $fetch['u_id'] = $fetchTime['u_id'];
        }
?>
<tr>
    <td><a href="<?php echo $website_url . 'thread/' . $fetch['t_id']; ?>" style="font-weight: bold;"><?php echo $fetch['title']; ?></a></td>
    <td><?php echo $rfetch[0]; ?></td>
    <td>
        <?php
            $monthNum  = substr($fetch['postdate'], 5, 2);
            $dateObj   = DateTime::createFromFormat('!m', $monthNum);
            echo $fetch['title'] . '<br />';
            echo substr($fetch['postdate'], 11, 8) . ' on ' . substr($fetch['postdate'], 8, 2) . ' ' . $dateObj->format('F') . ' ' . substr($fetch['postdate'], 0, 4) . '<br />';
            echo'by ' . $fetch['u_id'];
        ?>
    </td>
</tr>
<?php
    }
?>

I hope this makes any sense, it is hard to explain.

I already searched a bit and I found that I might need to use join, but I have no idea how I should do that.

---

Partial solution:

SELECT th.*, (count(po.t_id) + count(th.t_id)) AS amount, max(po.postdate) AS lastdate FROM thread th INNER JOIN threadpost po ON th.t_id = po.t_id GROUP BY po.t_id ORDER BY lastdate DESC

Only now it only displays a thread if there are also posts in it.

Answer 1

You don't need to loop through and can probably do this in a single SQL statement using WHERE EXISTS like

SELECT * 
FROM threadpost t1 
WHERE EXISTS (SELECT 1 FROM thread WHERE t_id = t1.t_id)
ORDER BY postdate DESC;

(OR) probably using a simple join query like

SELECT t1.* 
FROM threadpost t1 JOIN thread t2 ON t1.t_id = t2.t_id
ORDER BY t1.postdate DESC;