如何合并这些循环的MySQL查询？

Question

I've recently began a comments/replies section for an app.
My initial approach has been to query the database for the comments linked to a particular item as below:-

$commentquery = "SELECT projects_comments.*, users.user_url, users.display_name
                   FROM ".$wpdb->prefix."projects_comments projects_comments
              LEFT JOIN ".$wpdb->prefix."users users on users.ID=projects_comments.userid
                  WHERE projectid = '$projectid'
               ORDER BY projects_comments.commentid DESC";  

I then began a foreach loop with the following query inside in order to retrieve the replies:

if($comments) {
  foreach ( $comments as $c ) { 
    $replyquery = "SELECT project_replies.*, users.user_url, users.display_name
                     FROM ".$wpdb->prefix."project_replies project_replies
                LEFT JOIN ".$wpdb->prefix."users users on users.ID=project_replies.uid
                    WHERE project_replies.cid = '$c->commentid' 
                 ORDER BY project_replies.id DESC"; 
  }
}

In practice this is all working fine, however it strikes me that this is terribly inefficient and that there must be a way to retrieve all the data in one query. My concern is that with 100 comments on a page I will be performing 100 queries for the comments alone.

What is the best method to tackle this and how would I go about combining these queries to produce one large object/array with which to pull the data from.

Or alternatively, how can I at least prevent the 'query-within-the-loop'.

Answer 1

What I tend to do to avoid this setup is this:

SELECT ...
WHERE `keyfield` IN (".implode(",",$keyvalues).")

Answer 2

SELECT c.*, cu.user_url AS cu_url, cu.display_name AS cu_name,
       r.*, ru.user_url AS ru_url, ru.display_name AS ru_name
  FROM projects_comments AS c
  LEFT JOIN users        AS cu ON cu.ID = c.userid
  JOIN project_replies   AS r  ON r.cid = c.commentid 
  LEFT JOIN users        AS ru ON ru.ID = r.uid
 WHERE c.projectid = $projectid
 ORDER BY c.commentid DESC, r.id DESC

The only detail to worry about is the JOIN instead of possibly using LEFT JOIN when connecting Project_Replies and Project_Comments. This will only pick up comments that have at least one reply; using a LEFT JOIN, you'll get comments that have no replies too.

Answer 3

You can JOIN them like you do the users table

    $commentquery = "SELECT projects_comments.*, project_replies.*, users.user_url, users.display_name, users2.user_url as comment_user_url, users2.display_name as comment_display_name
                     from ".$wpdb->prefix."projects_comments projects_comments
                LEFT JOIN ".$wpdb->prefix."project_replies project_replies ON project_replies.cid = projects_comments.comment_id
                LEFT JOIN ".$wpdb->prefix."users users on users.ID=project_replies.uid
                left join ".$wpdb->prefix."users users2 on users2.ID=projects_comments.userid
                    WHERE projectid = '$projectid'  
                 ORDER BY projects_comments.commentid desc, project_replies.id DESC";