select field information with min time value

I thought I was doing the right thing but I keep getting the wrong result. I am trying to simply find the id of the entry with the min time, but I am not getting that entry.

$qryuserscount1="SELECT id,min(entry_time) FROM scrusersonline WHERE topic_id='$topic_id'";
$userscount1=mysql_query($qryuserscount1);
while ($row2 = mysql_fetch_assoc($userscount1)) {
    echo $onlineuser= $row2['id'];
}

That is my query, and it does not work. This however does work which does not make sense to me SELECT id FROM scrusersonline WHERE topic_id=’$topic_id’ ORDER by entry_time LIMIT 1, can anyone quickly point out what I am doing wrong?

Answer

SELECT id,min(entry_time) FROM scrusersonline WHERE topic_id='$topic_id'

will return id and entry_time for every row matching topic_id, since there is no group-by clause from which the min(entry_time) could be selected. The SELECT clause only defines what columns are to be returned, it doesn’t restrict that selection by rows any – that’s what the WHERE clause is for.

SELECT id FROM scrusersonline WHERE topic_id='$topic_id' ORDER BY entry_time LIMIT 1

will return id (SELECT id) for only the first (LIMIT 1) matching topic_id (WHERE topic_id='$topic_id') when ordered by entry_time (which is the minimum).

So, yes, SELECT id FROM scrusersonline WHERE topic_id='$topic_id' ORDER BY entry_time LIMIT 1 is what you’re looking for.