How to retrieve all posts with all comments using 2 SQL queries - php

Question

I am trying to make a page with a list of posts, and underneath each post all the comments belonging to that post. Initially I wanted to use just one query to retrieve all the posts + comments using the SQL's JOIN, but I find it impossible to for example retrieve a post with multiple comments. It only displays the posts with a maximum of 1 comment per post, or it just show a post multiple times depending on the amount of comments.
In this related question, somebody talked about using 2 queries:
How to print posts and comments with only one sql query
But how do I do this?
I've got the query and a while loop for posts, but I obviously don't want to run a query for comments for each post inside that loop.
$getPost = mysql_query('SELECT p.post_id,
p.user_id,
p.username,
p.content
FROM post p
ORDER BY p.post_id DESC');
while($row = mysql_fetch_array($getPost))
{
...
}
Table structure (reply is the table for storing comments):
POST (post_id (primary key), user_id, username, content, timestamp)
REPLY (reply_id (primary key), post_id, username, reply_content, timestamp)

Answer 1

You can do it in a single query, which is OK if the amount of data in your original posts is small:
$getPost = mysql_query('SELECT
p.*,
r.reply_id, r.username r_username, r.reply_content, r.timestamp r_timestamp
FROM post p
left join reply r
ORDER BY p.post_id DESC'
);
$posts = array();
$last_id = 0;
while($row = mysql_fetch_array($getPost))
{
if ($last_id != $row['post_id']) {
$posts[] = array(
'post_id' => $row['post_id'],
'user_id' => $row['user_id'],
'username' => $row['username'],
'content' => $row['content'],
'timestamp' => $row['timestamp'],
'comments' => array()
);
}
$posts[sizeof($posts) - 1]['comments'][] = array(
'reply_id' => $row['reply_id'],
'username' => $row['r_username'],
'reply_content' => $row['reply_content'],
'timestamp' = $row['r_timestamp']
);
}
Otherwise, break it into two queries like so:
$getPost = mysql_query('SELECT
p.*,
FROM post p
ORDER BY p.post_id DESC'
);
$rows = array();
$ids = array();
$index = array();
while($row = mysql_fetch_assoc($getPost)) {
$row['comments'] = array();
$rows[] = $row;
$ids[] = $row['post_id'];
$index[$row['post_id']] = sizeof($rows) - 1;
}
$getComments = mysql_query('select r.* from replies r where r.post_id in ("'
. join('","', $ids)
. '")');
while ($row = mysq_fetch_assoc($getComments)) {
$rows[$index[$row['post_id']]]['comments'][] = $row;
}
... Or something like that. Either option allows you to litter your first query with WHERE clauses and so forth to your heart's content. The advantage of the 2nd approach is that you don't re-transmit your original post data for each comment!

Answer 2

In order to also get those posts without comments, you need to use a LEFT OUTER JOIN. In that case, any row in the first table without any corresponding rows in the second table will be paired with a row consisting of null values.
SELECT * FROM posts
LEFT OUTER JOIN comments ON posts~post_id = comments~post_id;
By the way: there is also a RIGHT OUTER JOIN. When you would use that, you would get all comments, including those where the parent post got lost somehow, but no posts without comments.