如何从选择中选择？

Question

I'm creating a commenting system, which will have 2 top comments.

How can I select the latest 20 rows, and then from that selection, select the top 2 rows (likes-dislikes)? I can do it with a PHP loop, but it would not be as efficient. Currently I am just selecting the top 2 from the all the comments, but the two top comments never change, since people just up-vote those ones:

SELECT * FROM pagecomments WHERE page_id='$pageid' ORDER BY likes-dislikes DESC LIMIT 2

EDIT: The table is ordered by the the column "id", which is auto_increment. page_id is the page on the site. Sorry.

Answer 1

Nest your query:

SELECT   *
FROM (
  SELECT   *
  FROM     pagecomments
  WHERE    page_id='$pageid'
  ORDER BY date DESC
  LIMIT    20
) t
ORDER BY likes-dislikes DESC
LIMIT    2

Answer 2

Order not only by LIkes, first order by date entered or timestamp. By ordering by date, you assure that you`ll get the latest 20 posts or comments.

SELECT * FROM pagecomments WHERE page_id='$pageid' ORDER by date_entered desc , 
likes-dislikes DESC limit 2

Answer 3

Since your id column is set to auto_increment , use it in a subquery:

select *, likes-dislikes
from (
  select *
  from pagecomments
  where page_id='$pageid'
  order by id desc
  limit 20
  ) t
order by likes-dislikes desc
limit 2

• Condensed SQL Fiddle Demo