Get latest 3 comments for a list of posts

Currently I'm running one query to get all posts for a user, then in the loop for that, I am querying for the latest 3 comments for that particular post. Super inefficient; I'm querying over and over again for every post.

I would like to consolidate my queries so that I query just once for all posts, and just once for all comments for those particular posts. At the moment I have a comma-separated list that I made for all posts for this user (e.g. "1,5,18,9")

posts table:

• posts.id
• posts.userid

comments table:

• comments.id
• comments.relid (this is the postid)
• comments.userid

The query should use the $posts_list I have, which is the comma-separated list of posts. Or a subselect for all posts for this user, bu开发者_开发知识库t that seems inefficient since I already have the post list in a string.

Thanks so much for any help!

Try this query -

SELECT p.id, p.userid, c.id, c.userid
  FROM posts p
  JOIN (
    SELECT c1.*, COUNT(*) rank FROM comments c1
    LEFT JOIN comments c2
      ON c2.relid = c1.relid AND c2.id <= c1.id
    GROUP BY c1.relid, c1.id
    ) c
  ON p.id = c.relid
WHERE rank < 4

And add condition you need, i.e. - WHERE p.userid IN (1,5,18,9).