Counting the number of rows returned from db_query() with the "SELECT" statement
If you look at the db_result() documentation for Drupal 6:

function db_result($result) {
   if ($result &amp;&amp; mysql_num_rows($result) &gt; 0) {
    // The mysql_fetch_row function has an optional second parameter $row
    // but that can't be used for compatibility with Oracle, DB2, etc.
    $array = mysql_fetch_row($result);
    return $array[0];
   }
   return FALSE;
}


I also saw that you could do this in Drupal 6:

$num_rows = db_result(
  db_query("SELECT COUNT(*) FROM {node} WHERE type = '%s'", $type-&gt;type)
);


It looks like you could simply do:

// Execute your query.
$result = db_query($your_query);
// Use mysql_num_rows() on the result set.
$num_rows = mysql_num_rows($result);