สอบถามปัญหาคำสั่ง SQLครับ มี2ตารางต้องการแสดงคอมเม้นล่าสุดทำยังไงครับ

create table if not exists board(
  id_board int unsigned not null auto_increment primary key,
  topic varchar(50) not null
  )engine=myisam default character set utf8;
  
  create table if not exists txtcomment(
  id_comment int unsigned not null auto_increment primary key,
  id_board int unsigned not null,
  postdate datetime null default null,
  txt_comment varchar(100) not null
  )engine=myisam default character set utf8;
  
  insert ignore into board values(null,'Hello.'),(null,'Where are you come from?'),
(null , 'Rides for life.');

insert ignore into txtcomment values(null,1,'2015-06-20 12:32:30','hello world!'),
(null,1,'2015-06-21 8:30:10','hello,how areyou?'),
(null,2,'2015-06-10 12:12:30','I come from USA.'),
(null,2,'2015-07-01 23:45:10','No matter where I am from.'),
(null,2,'2015-07-01 12:32:30','I am from Taiwan Taipae.'),
(null,2,'2015-07-02 09:12:41','I lives in Thailand so long times.'),
(null,3,'2015-06-11 04:47:10','Yes,I love to rides.Could you suggest?'),
(null,3,'2015-06-17 14:12:30','Can I join the trip?');

// จากข้อมูลทดสอบด้านบน เราจะเขียนคำสั่งได้ดังนี้

select t1.id_board,t1.topic,t2.id_comment,t2.txt_comment,t2.postdate
from board t1 left join txtcomment t2 using(id_board)
where concat(postdate,':',id_board) in (select concat(max(postdate),':',id_board) from txtcomment group by id_board order by postdate desc)
order by t1.id_board,postdate desc 

select t1.id_post,t1.topic,t2.id_comment,t2.timecom
from data_board t1 left join data_comment t2 using(id_post)
where concat(timecom,':',id_post) in (select concat(max(timecom),':',id_post) from data_comment group by id_post order by timecom desc)
order by t1.id_post,timecom desc 

select t1.id_post,t1.topic,t2.id_comment,t2.timecom
from data_board t1 
inner join (
    select c1.* from data_comment c1 
    inner join ( 
         select id_post, max(timecom) maxtime 
         from data_comment group by id_post
    ) c2 on c1.id_post=c2.id_post and c1.timecom=c2.maxtime
) t2 using(id_post)
order by timecom desc,t1.id_post 

select * from board
left join 
(select comment.* from comment 
	inner join (select max(id_comment) as m_id_comment,id_board  from comment group by id_board) as xx on xx.m_id_comment=comment.id_comment
) as yy on yy.id_board=board.id_board