DROP TABLE IF EXISTS comments;
CREATE TABLE comments (
  id  int(11) NOT NULL AUTO_INCREMENT,
  parent int(11) NOT NULL,
  login varchar(15) NOT NULL,
  note varchar(255) NOT NULL,
  PRIMARY KEY(id,parent),
  KEY(parent)
  );
INSERT INTO comments VALUES 
(1,0,'user1','как погода?'),
(2,1,'user2','погода норм.'),
(3,0,'user4','всем привет'),
(4,1,'user3','говорят зима теплая'),
(5,2,'user1','хорошая?');

SELECT IF(parent,'-',''),c.parent,c.login,note
FROM(
SELECT @index:=INSTR(@a,',') as coma_index,
@id:=SUBSTRING_INDEX(@a, ',', 1)  as id,
@a:=SUBSTRING(@a,IF(@index,@index+1,0)),
@a:=CONCAT_WS(IF(LENGTH(@a)>0,',',''),(SELECT group_concat(id ORDER BY id) FROM comments c WHERE c.parent=@id),@a) as array,
@sort:=@sort+1 AS sort
FROM comments c,(SELECT@a:= GROUP_CONCAT(id),@sort:=0 FROM comments WHERE parent =0) init
WHERE LENGTH(@a)>0
) AS hi 
JOIN comments c ON hi.id=c.id
ORDER BY sort

SELECT IF(parent,'-',''),c.parent,c.login,note
FROM(
SELECT @index:=INSTR(@a,',') as coma_index,
@id:=SUBSTRING_INDEX(@a, ',', 1)  as id,
@a:=SUBSTRING(@a,IF(@index,@index+1,0)),
@a:=CONCAT_WS(IF(LENGTH(@a)>0,',',''),(SELECT group_concat(id ORDER BY id) FROM comments c WHERE c.parent=@id),@a) as array,
@sort:=@sort+1 AS sort
FROM comments c,(SELECT@a:= GROUP_CONCAT(id ORDER BY id),@sort:=0 FROM 
(SELECT id FROM comments WHERE parent =0 ORDER BY id LIMIT 0,1) AS limited_roots #лимитить - листать рутов
                  ) as init
WHERE LENGTH(@a)>0
) AS hi 
JOIN comments c ON hi.id=c.id
ORDER BY sort