Combine three SQL queries - sql

Question

I'm trying to fetch data for my forum's index. Fetching a list of all the boards, the number of threads in that board, and the number of posts for each of those threads in that board.
SELECT
board.*,
IFNULL(a.thread_count, 0) AS thread_count,
b.post_count
FROM
(SELECT * FROM r_forum_boards ORDER BY position) board
LEFT OUTER JOIN
(SELECT r_forum_threads.board, r_forum_threads.id,
COUNT(r_forum_threads.id) AS thread_count
FROM r_forum_threads) a
ON board.id = a.board
LEFT OUTER JOIN
(SELECT r_forum_posts.thread_id, COUNT(*) AS post_count
FROM r_forum_posts) b
ON b.thread_id = a.id
The problem is that post_count is returning NULL. I've tried a few different variations of this, but none of them are working.

Answer 1

I'm guessing from the IFNULL that your SQL is MySQL-flavored. In that case, you can use COUNT DISTINCT to simplify things.
SELECT
board.id,
COUNT(DISTINCT r_forum_threads.id) AS thread_count,
COUNT(r_forum_posts.id) AS post_count
FROM board
LEFT OUTER JOIN r_forum_threads ON board.id = r_forum_threads.board
LEFT OUTER JOIN r_forum_posts ON r_forum_posts.thread_id = r_forum_threads.id
GROUP BY board.id
ORDER BY board.position
Depending on how much of board.* you actually need, either add columns to the SELECT and GROUP or use this as a subquery to join back to board.

Answer 2

Try putting in a GROUP clause:
LEFT OUTER JOIN
(SELECT r_forum_threads.board, r_forum_threads.id, COUNT(r_forum_threads.id) AS thread_count FROM r_forum_threads GROUP BY r_forum_threads.id) a
ON board.id = a.board
LEFT OUTER JOIN
(SELECT r_forum_posts.thread_id, COUNT(*) AS post_count FROM r_forum_posts GROUP BY r_forum_posts.thread_id) b ON b.thread_id = a.id
See if that does the trick.

Answer 3

Perhaps because you are missing the Group By clause in your subqueries? In addition, you do not need the first subquery.
Select board...
, Coalesce(a.thread_count, 0) AS thread_count
, b.post_count
From r_forum_boards
Left Join (
Select r_forum_threads.board
, r_forum_threads.id
, Count(r_forum_threads.id) AS thread_count
From r_forum_threads
Group By r_forum_threads.board
, r_forum_threads.id
) a
On a.board = board.id
Left Join (
Select r_forum_posts.thread_id
, Count(*) AS post_count
From r_forum_posts
Group By r_forum_posts.thread_id
) As b
On b.thread_id = a.id
Order By r_forum_boards.position
You might consider changing the query slightly to make it easier to test:
Select board...
, Coalesce(a.thread_count, 0) AS thread_count
, A.post_count
From r_forum_boards
Left Join (
Select r_forum_threads.board
, r_forum_threads.id
, Count(r_forum_threads.id) AS thread_count
, Posts.post_count
From r_forum_threads
Left Join (
Select r_forum_posts.thread_id
, Count(*) AS post_count
From r_forum_posts
Group By r_forum_posts.thread_id
) As Posts
On Posts.thread_id = r_forum_threads.Id
Group By r_forum_threads.board
, r_forum_threads.id
) As A
On A.board = board.id
Order By r_forum_boards.position
In this way, you can run the single inner query and ensure you A. get results and B. get values for post_count.

Answer 4

The problem I see is that you're trying to get 2 related but slightly conflicting pieces of data, and probably 2 queries will get you what you need.
You first need a query to get the board names and the number of threads in each board.
Select Board.*, GroupThread.threadCount
FROM r_forum_boards Board
INNER JOIN (Select board_id, count(*) as threadCount from r_forum_threads group by board_id) GroupThread ON Board.board_id = GroupThread.board_id
Second, for each thread, you need the posts, which are calculated in basically the same way:
Select Thread.*, GroupPosts.postCount
FROM r_forum_threads Thread
INNER JOIN (Select thread_id, count(*) as postCount from r_forum_posts group by thread_id) GroupPosts ON Thread.board_id = GroupPosts.thread_id
In each of these cases, you look at the parent object, and count the children.