Question

I’m working on a blogging app that requires a unique query.

Problem: I need to display one parent post, all it’s children posts (up to a certain number before requiring pagination), and up to 5 comments associated with each child post, and the parent.

I wrote this query, but it doesn’t work because it will return only 5 comments that belong to the parent post.

SELECT
    posts.id, posts.postTypeId, posts.parentId, posts.ownerUserId, posts.body
, users.id AS authorId, users.displayname AS authorDisplayName
, comments.id AS commentId, comments.text AS commentText
, comments.commentOwnerUserId, comments.commentOwnerDisplayName
FROM posts
JOIN users ON posts.owneruserid = users.id
LEFT JOIN ( SELECT comments.id, comments.postId, comments.text, commenters.id AS commentOwnerUserId, commenters.displayname AS commentOwnerDisplayName
        FROM comments
        JOIN users AS commenters ON comments.userid = commenters.id
        ORDER BY comments.createdat ASC
        LIMIT 0,5 ) AS comments ON comments.postid = posts.id
WHERE posts.id = @postId OR posts.parentId = @postId
ORDER BY posts.posttypeid, posts.createdAt

The query returns the parent post, all it’s children, and the first 5 comments it encounters, (usually they belong to the parent because we are ordering by postTypeId, and the parent is the first post). If the first post doesn’t have 5 comments, it moves on the next post and returns those comments, until the 5 limit is reached.

What I need is to return one parent post and all it’s children posts, and up to 5 comments for each child, the parent. I also need the owner data for each post and comment.

UPDATE I’m open to doing this with more than one query if it will scale well. The only condition is that the parent and children posts retrieval happens in the same query.

Any idea how I can write such a query? I included my schema below.

/* Posts table */
CREATE TABLE `posts` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `posttypeid` int(10) NOT NULL,
  `parentid` int(10) DEFAULT NULL,
  `body` text NOT NULL,
  `userid` int(10) NOT NULL,
  `createdat` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
KEY `parentId` (`parentid`)
KEY `userId` (`userid`)
) ENGINE=InnoDB AUTO_INCREMENT=572 DEFAULT CHARSET=utf8

/* Comments table */
CREATE TABLE `comments` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `postid` int(10) NOT NULL,
  `userid` int(10) NOT NULL,
  `text` text NOT NULL,
  `createdat` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `postId` (`postid`),
  KEY `userId` (`userid`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8

/* users table */
CREATE TABLE `users` (
  `id` int(10) NOT NULL AUTO_INCREMENT,
  `email` varchar(50) NOT NULL,
  `displayname` varchar(50) NOT NULL,
  `createdat` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `email` (`email`),
) ENGINE=InnoDB AUTO_INCREMENT=66 DEFAULT CHARSET=utf8

Answer 1

I take as granted that every child has exactly one parent. Then, I think this will work:

SELECT p.*             <-- post details
     , u.*             <-- user details
     , cc.*            <-- comment details
FROM 
  ( ( SELECT parentid AS id
      FROM posts
      WHERE posts.id = @mypostid     <-- the id of the post we want 
    )
    UNION ALL
    ( SELECT child.id
      FROM posts AS parent
        JOIN posts AS child
          ON child.parentid = parent.id
      WHERE parent.id = 
            ( SELECT posts.parentid
              FROM posts
              WHERE posts.id = @mypostid)   <-- the id of the post we want 
      ORDER BY child.createdat            <-- any order you prefer
      LIMIT x, 5                          <-- 5 children posts
    )
  ) AS pp
  JOIN posts p
    ON p.id = pp.id
  JOIN users
    ON users.id = p.userid
  JOIN comments cc
    ON cc.postid = pp.id
WHERE cc.postid IN
  ( SELECT c.id
    FROM comments c
    WHERE c.postid = pp.id
    ORDER BY c.createdat             <-- any order you prefer
    LIMIT y, 5                       <-- 5 comments for every post
  )

The x,5 should be replaced with 0,5 for first five childen posts and y,5 with 0,5 for first five comments. Then with 5,5 for next five, 10,5 for next five, etc.

---

UPDATE

Sorry, my mistake. The above gives the error:

This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'

I’ll wrap my head up to work around this 🙂