Question

this oughta be an easy one.

My question is very similar to this one; basically, I’ve got a table of posts, a table of comments with a foreign key for the post_id, and a table of votes with a foreign key for the post id. I’d like to do a single query and get back a result set containing one row per post, along with the count of associated comments and votes.

From the question I’ve linked to above, it seems that for getting a table back containing just a row for each post and a comment count, this is the right approach:

SELECT a.ID, a.Title, COUNT(c.ID) AS NumComments
FROM Articles a
LEFT JOIN Comments c ON c.ParentID = a.ID
GROUP BY a.ID, a.Title

I thought adding vote count would be as easy as adding another left join, as in

SELECT a.ID, a.Title, COUNT(c.ID) AS NumComments, COUNT(v.id AS NumVotes)
FROM Articles a
LEFT JOIN Comments c ON c.ParentID = a.ID
LEFT JOIN Votes v ON v.ParentID = a.ID
GROUP BY a.ID, a.Title

but I’m getting bad numbers back. What am I missing?

Answer 1

SELECT 
  a.ID, 
  a.Title, 
  COUNT(DISTINCT c.ID) AS NumComments, 
  COUNT(DISTINCT v.id) AS NumVotes
FROM 
  Articles           a
  LEFT JOIN Comments c ON c.ParentID = a.ID
  LEFT JOIN Votes    v ON v.ParentID = a.ID
GROUP BY 
  a.ID, 
  a.Title