Forum MySQL queries

Question

I am building a basic forum with CodeIgniter and I am having trouble with two MySQL queries.

Here are my tables:

mysql> describe forum_posts;
+----------+------------------+------+-----+-------------------+----------------+
| Field    | Type             | Null | Key | Default           | Extra          |
+----------+------------------+------+-----+-------------------+----------------+
| id       | int(11) unsigned | NO   | PRI | NULL              | auto_increment |
| topic_id | int(11)          | YES  | MUL | NULL              |                |
| user_id  | int(11)          | YES  | MUL | NULL              |                |
| post     | text             | YES  |     | NULL              |                |
| date     | timestamp        | YES  |     | CURRENT_TIMESTAMP |                |
| ip       | varchar(15)      | YES  |     | NULL              |                |
+----------+------------------+------+-----+-------------------+----------------+
6 rows in set (0.01 sec)

mysql> describe forum_topics;
+-------------+------------------+------+-----+---------+----------------+
| Field       | Type             | Null | Key | Default | Extra          |
+-------------+------------------+------+-----+---------+----------------+
| id          | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| category_id | int(11)          | YES  | MUL | NULL    |                |
| user_id     | int(11)          | YES  | MUL | NULL    |                |
| name        | varchar(100)     | YES  |     | NULL    |                |
| importance  | tinyint(2)       | YES  |     | 0       |                |
+-------------+------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

mysql> describe forum_categories;
+-------+------------------+------+-----+---------+----------------+
| Field | Type             | Null | Key | Default | Extra          |
+-------+------------------+------+-----+---------+----------------+
| id    | int(11) unsigned | NO   | PRI | NULL    | auto_increment |
| name  | varchar(50)      | YES  |     | NULL    |                |
| order | tinyint(2)       | YES  |     | 0       |                |
+-------+------------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)

1) I want to select all categories and their respective latest posts
2) I want to select all topics from category with id X and show the last post whitin this topic.

This is what I've done so far

(1):

SELECT 
    C.*,

    T.name AS topic_name, 
    T.id AS topic_id,

    P.date AS post_date

FROM forum_categories AS C

LEFT JOIN 
    (
        SELECT id, category_id, name
        FROM forum_topics
        ORDER BY id DESC
    ) 
    T ON C.id = T.category_id

LEFT JOIN 
    (
        SELECT id, topic_id, date
        FROM forum_posts
        ORDER BY DATE(date) DESC
    ) 
    P ON T.id = P.topic_id

GROUP BY C.id
ORDER BY DATE(P.date) DESC

(2):

SELECT 
    T.*, 
    P.topic_id,
    P.id AS post_id,
    P.user_id,
    P.date,
    users.username
FROM forum_topics AS T
LEFT JOIN 
    (
        SELECT id, topic_id, user_id, date
        FROM forum_posts
        ORDER BY id DESC
    )
    P ON P.topic_id = T.id
LEFT JOIN 
    users ON P.user_id = users.id
WHERE T.category_id =  '1'
GROUP BY P.topic_id
ORDER BY 
    T.id DESC,
    P.id DESC
LIMIT 10

Basically my problem is that I cannot first ORDER the subqueries and then GROUP them.

Also I am not sure if these quesries are efficient enough. I am working about the subqueries. Are they gonna be slow?

Answer 1

After many hours of breaking my head into the wall I think I found the solution for (1) and now it will be easier to find solution for (2).

SELECT 
    C.id,
    C.name AS category_name,
    C.order AS category_order,
    T.*

FROM forum_categories AS C

LEFT JOIN 
    (
        SELECT 
            T.category_id, 
            T.id AS topic_id, 
            T.name AS topic_name,
            P.user_id,
            P.username,
            P.date
        FROM forum_topics AS T

        LEFT JOIN
            (
                SELECT 
                    P2.id AS post_id,
                    P2.topic_id,
                    P2.user_id,
                    P2.date, 
                    U.username
                FROM forum_posts AS P2
                LEFT JOIN users AS U ON P2.user_id = U.id
                ORDER BY P2.id DESC
            )
            P ON P.topic_id = T.id

        ORDER BY P.post_id DESC
    )
    T ON T.category_id = C.id

GROUP BY C.id

Edit: Fixed a little bug in the (1) query and here is the (2) for those which want to accomplish the same as me:

SELECT 
    T.*,
    P.*

FROM forum_topics AS T

LEFT JOIN 
    (
        SELECT 
            P2.id AS post_id,
            P2.topic_id, 
            P2.user_id,
            P2.date,
            U.username
        FROM forum_posts AS P2
        LEFT JOIN users U ON U.id = P2.user_id
        ORDER BY P2.id DESC 
    )
    P ON P.topic_id = T.id 

WHERE T.category_id =  '1'
GROUP BY P.topic_id
ORDER BY P.topic_id DESC

Answer 2

Without being able to test, this is an idea i would try to start with. It may not be how others would do it, but the logic should at least help point you in a direction. I'm sure the critics will come out but ya know, no one is answering this so i figure i would try to get you going in a direction!

SELECT *
FROM forum_categories
$catArray ^^

SELECT p.*, t.*
FROM forum_posts p, forum_topics t, forum_categories c
WHERE t.category_id = c.id
AND p.topic_id = t.id
ORDER BY c.order ASC, p.date DESC
GROUP BY t.id
$postArray^^

for ($x = 0; $x < count($postArray); $x++)
{
    for ($y = 0; $y < count($catArray); $y++)
    {
        if ($catArray[$y]['id'] == $postArray[$x]['category_id'])
        {
            $catData = $catArray[$y];
            break;
        }
    }

    if ($catData['id'] != $currentCat)
    {
        PRINT CATEGORY >> $catData['name']
    }

    PRINT TOPIC >> $postArray[$x]['name']
    PRINT NEWEST POST >> $postArray[$x]['post'] 

    $currentCat = $catData['id'];
}