Question

I have three tables underlying a blog: one for users, one for comments and one for threads.
Not all the records retrieved are echoed by PHP and I think it may be to do with NULL values, for example on my memberlist page the following query retrieves data on four users:

SELECT COUNT(Topics.MemberID) AS NumberOfTopics, Users.id, 
    Users.FirstName, Users.LastName, Users.Joined 
FROM Users LEFT JOIN Topics ON Users.id=Topics.MemberID GROUP BY Topics.MemberID

However if the last user(s) at the end of the list has no topics (NumberOfTopics = 0) they are not presented.
If a user has a post count of 0 but they following record has a post count that isn’t 0 the user is shown. Why?

Here is the complete code:

$result = mysql_query("SELECT COUNT(Topics.MemberID) AS NumberOfTopics, ".
                      "Users.id, Users.FirstName, Users.LastName, Users.Joined ".
                      "FROM Users LEFT JOIN Topics ON Users.id=Topics.MemberID ".
                      "GROUP BY Topics.MemberID") 
             or die("Query failed with error: ".mysql_error());

echo "<table border='1'>
<tr>
<th>Name</th>
<th>Joined</th>
<th>No. of posts</th>
</tr>";

while($row = mysql_fetch_assoc($result))
  {
  echo "<tr>";
  echo "<td><a href='/neuro/profile.php?userid=$row[id]'>" . 
       $row['FirstName'] . " " . $row['LastName'] . "</a></td>";
  echo "<td>" . $row['Joined'] . "</td>";
  echo "<td>" . $row['NumberOfTopics'] . "</td>";
  echo "</tr>";
  }
echo "</table>";

mysql_close($con);

Thanks for reading!

Answer 1

I believe the problem is that your GROUP BY will force the elimination of NULLs.

I normally will use subquery in these circumstances simply because it is easier to read and understand:

SELECT (select count(*) from Topics T where T.MemberID = U.id) AS NumberOfTopics,
    Users.id, Users.FirstName, Users.LastName, Users.Joined 
FROM Users u