MySQL SELECT Statement Only Returns One Row

Question

It's been quite a long time since I last asked a question, but I've been stumped messing with SQL in an attempt to build my own forum system.

The background of my problem is that I'm trying to create a function to display the most popular threads based on how many replies it has and how recent the latest reply was.

With some reading, I made this SQL statement.

SELECT 
topics.id, 
topics.authorID,
topics.lastReply
FROM 
ms_topics as topics
INNER JOIN
(SELECT inTopic FROM ms_posts) as rcount
ON
rcount.inTopic = topics.id
ORDER BY
topics.lastReply DESC,
count(rcount.inTopic)

And then I use PDO's fetchAll() function to bring the results into a variable.

$GetPopular = $db->prepare('SQL Statement Above');
$GetPopular->execute();
$Populars = $GetPopular->fetchAll();

And, in an effort to trouble shoot, I use var_dump() on $populars , which returns only one row from the database.

array(1) { [0]=> array(3) { ["id"]=> string(1) "4" ["authorID"]=> string(1) "1" ["lastReply"]=> string(19) "2014-06-08 19:49:50" } }

My question is basically this; why would it not be returning more than one row, and how do I make it do so?

Answer 1

You are on the right track with the joined subquery, but you need to apply the COUNT() inside the subquery. Otherwise, the aggregate group is applied to the outer query, resulting in one row.

This is because you have no GROUP BY , which MySQL is lenient about. It gives you one row back with not quite the results you expect, where most other RDBMS would give you a parse error. Otherwise, an aggregate function ( COUNT(),MIN(),MAX(),SUM() , etc) with no GROUP BY will never result in more than one row returned.

SELECT 
  topics.id, 
  topics.authorID,
  topics.lastReply,
  postsInTopic
FROM 
  ms_topics as topics
  /* The subquery provides the aggregate count
     grouped per inTopic */
  INNER JOIN (
    SELECT
     inTopic,
     COUNT(*) AS postsInTopic
   FROM ms_posts
   GROUP BY inTopic
  ) as rcount ON rcount.inTopic = topics.id
ORDER BY
  topics.lastReply DESC,
  /* Use the alias from the subquery postsInTopic to srot */
  postsInTopic

A quick note about PDO and prepare() : While it is a good thing that it's in your workflow, it is not strictly necessary to use prepare()/execute() if your query has no input values (no dynamic WHERE clause, for example). For a static query in a case like this, you can safely call PDO::query() with less overhead and a little less code.