<?php
$comment = _comment_load(12);
return theme('comment',$comment);
?>

<?php
  $node = node_load(5);
 print comment_render($node);
?>

<?php
  $nid = 5;
  db_query('SELECT * FROM {comments} WHERE nid = %d', $nid);
?>

$result = db_select('comment')
  ->fields('comment', array('cid'))
  ->condition('nid', $node->nid, '=')
  ->execute();

$cids = $result->fetchCol();
$comments=comment_load_multiple($cids);

foreach($comments as $comment) {
  //do some stuff with the comment
  print $comment->comment_body ["und"][0] ["value"];
}

$result = db_select('comment')
                  ->fields('comment', array('name','subject'))
                  ->condition('nid', $node->nid, '=')
                  ->execute()
                  ->fetchAssoc();
foreach($result as $comment) {
   print $comment;
 }