Question

I have MySQL query which I think needs a subquery. I’d like to count the total number of “up” votes on each of many comments and determine whether a given user has already voted on each comment:

Here are my tables:

Comments Table:
  comment_id  comment      acct_id     topic_id    comment_date
      5        hello5        2             1            9:00am
      7        hello7        3             1           10:00am

  Votes Table:
  comment_id   vote        acct_id     topic_id
     5          1             1            1
     7          1             4            1
     5          1             5            1  

here’s the output i’m getting:

  comment_id  commenter  comment  voter   sum    did_i_vote 
      5           2      hello5     1      2          1 
      7           3      hello7     4      1          1

Here’s the desired output:

  comment_id  commenter  comment  voter   sum    did_i_vote 
      5           2      hello5   **5**    2          1 
      7           3      hello7     4      1          1

Here’s my query:

SELECT votes.acct_id as voter, comments.comment_id, comment, comments.acct_id as 
commenter, SUM(vote) as sum, vote as did_i_vote
from votes 
right join comments 
on votes.comment_id=comments.comment_id 
join accounts on comments.acct_id=accounts.acct_id 
where topic_id=1 
group by comments.comment_id order by comment_date desc  

You’ll notice these 2 outputs are identical except for voter.

What my query is missing is a way to determine whether a given user, for example with voter=acct_id=5, was the one who voted on any of the comments. Without that condition, the query picks the first voter in the list which for comment_id=5 is voter=1.

So my question is I think I need to insert the following subquery:

SELECT from votes where voter='X'

I’m just not sure where or how. Putting it in parentheses in between the from and votes above eliminates the sum() function so I’m stuck.

Any thoughts?

Answer 1

If I’ve understood you correctly from your comments above, I think all you need to do is (outer) join the votes table to your query another time, this time only on the votes of the account in question:

SELECT
   comments.comment_id       AS comment_id,
   comments.acct_id          AS commenter,
   comment                   AS comment,
-- votes.acct_id             AS voter,                        -- ambiguous
   SUM(votes.vote)           AS sum,
   my_votes.vote IS NOT NULL AS did_i_vote
FROM
             votes
  RIGHT JOIN comments ON votes.comment_id=comments.comment_id
        JOIN accounts ON comments.acct_id=accounts.acct_id    -- what purpose ?
  LEFT  JOIN votes AS my_votes ON
               my_votes.commentid=comments.comment_id
           AND my_votes.acct_id=@my_acct_id
WHERE topic_id = 1                                            -- ambiguous
GROUP BY comments.comment_id
ORDER BY comment_date DESC