Question

I have been working at this one for an hour. I have tried several different joins, and subqueries with no luck. Here is the situation.

Two tables. One with an main index, and one a listing of votes from users. I want to determine how many votes a particular user is leaving for another user (easy)… THEN figuring out the percentage of the total votes and sorting by that (hard).

Table 1 has columns: post_id, poster_id
Table 2 has columns: post_id, voter_id, vote

The post_id is correlated between the two tables. A simple query like this will get you an output showing the total votes 1 user has left another user… then sorts showing who has left the most votes for another.

SELECT poster_id, voter_id, count(*) AS votes
FROM table_1, table_2
WHERE table_1.post_id = table_2.post_id
GROUP BY poster_id, voter_id
ORDER BY votes DESC 

That works great… but I want to see who is leaving the most votes as a percentage of the users total votes. So I need to ALSO get the total votes a “poster_id” has, then divide the current number into a percentage… then sort into that percentage. Output should be something like:

poster_id | voter_id | votes | vote_total | percent
----------------------------------------------------
   1      |    3     |   10  |     10     |   100%
   3      |    1     |   15  |     25     |    60%     
   2      |    1     |   3   |      6     |    50%
   2      |    3     |   2   |      6     |    33%
   3      |    2     |   5   |     25     |    20%
   2      |    4     |   1   |      6     |    17%

etc.

Basically voter #3 is responsible for 100% of poster #1’s votes. Poster #3 got 60% of its votes from voter #1… etc.

We’re trying to find out if there is a particular user giving someone more votes (as a percentage) than other users to try and find potential abuses.

I thought a RIGHT JOIN would work, but it is not working out.

SELECT t1.poster_id, t1.voter_id, count(*) AS votes, count(t3.*) AS votes_total, votes / votes_total AS percentage
FROM (table_1 t1, table_2 t2)
RIGHT JOIN (table_1 t3, table_2 t4)
    ON (t3.post_id = t4.post_id AND t3.poster_id = t1.poster_id)
WHERE t1.post_id = t2.post_id
GROUP BY t1.poster_id, t2.voter_id
ORDER BY percentage DESC 

Basically runs forever and doesn’t return anything. I typed that query from memory, and doesn’t exactly represent the real table names. Any points in the right direction would help. Inner join perhaps?

Answer 1

Try this:

SELECT
    poster_id,
    voter_id,
    count(*) AS votes,
    count(*) * 100 / total.total_votes as percentage
FROM table_1
join (
    select poster_id, count(*) AS total_votes
    FROM table_1
    join table_2 on table_1.post_id = table_2.post_id
    GROUP BY poster_id
) total on total.poster_id = table_1.poster_id
join table_2 on table_1.post_id = table_2.post_id
GROUP BY poster_id, voter_id
ORDER BY votes DESC

This uses a subquery to return the total votes for each poster_id then joins to it as if it were a table.

Note also the change to using proper joins instead of joining through the where clause.