My tables:
suggestions:
suggestion_id|title|description|user_id|status|created_time
suggestion_comments:
scomment_id|text|user_id|suggestion_id
suggestion_votes:
user_id|suggestion_id|value
Where value is the number of points assigned to a vote.
I'd like to be able to SELECT: suggestion_id, title, the number of comments and the SUM of values for that suggestion. sorted by SUM of values. LIMIT 30
Any ideas?
You may want to try using sub queries, as follows:
SELECT s.suggestion_id,
(
SELECT COUNT(*)
FROM suggestion_comments sc
WHERE sc.suggestion_id = s.suggestion_id
) num_of_comments,
(
SELECT SUM(sv.value)
FROM suggestion_votes sv
WHERE sv.suggestion_id = s.suggestion_id
) sum_of_values
FROM suggestions s;
Test case:
CREATE TABLE suggestions (suggestion_id int);
CREATE TABLE suggestion_comments (scomment_id int, suggestion_id int);
CREATE TABLE suggestion_votes (user_id int, suggestion_id int, value int);
INSERT INTO suggestions VALUES (1);
INSERT INTO suggestions VALUES (2);
INSERT INTO suggestions VALUES (3);
INSERT INTO suggestion_comments VALUES (1, 1);
INSERT INTO suggestion_comments VALUES (2, 1);
INSERT INTO suggestion_comments VALUES (3, 2);
INSERT INTO suggestion_comments VALUES (4, 2);
INSERT INTO suggestion_comments VALUES (5, 2);
INSERT INTO suggestion_comments VALUES (6, 3);
INSERT INTO suggestion_votes VALUES (1, 1, 3);
INSERT INTO suggestion_votes VALUES (2, 1, 5);
INSERT INTO suggestion_votes VALUES (3, 1, 1);
INSERT INTO suggestion_votes VALUES (1, 2, 4);
INSERT INTO suggestion_votes VALUES (2, 2, 2);
INSERT INTO suggestion_votes VALUES (1, 3, 5);
Result:
+---------------+-----------------+---------------+
| suggestion_id | num_of_comments | sum_of_values |
+---------------+-----------------+---------------+
| 1 | 2 | 9 |
| 2 | 3 | 6 |
| 3 | 1 | 5 |
+---------------+-----------------+---------------+
3 rows in set (0.00 sec)
UPDATE: @Naktibalda's solution is an alternative solution that avoids sub queries.
suggestion_id of each table is properly indexed, (and as long as the query won't return many suggestions)... In addition, it can get quite complicated to do multiple aggregations on a single result set.I was typing the same query as potatopeelings.
But there is an issue:
Resultset after joins contains M*N rows (M-number of comments, N-number of votes, not less than 1) for each suggestion.
To avoid that you have to count distinct comment ids and divide a sum of votes by number of comments.
SELECT
s.*,
COUNT(DISTINCT c.scommentid) AS comment_count,
SUM(v.value)/GREATEST(COUNT(DISTINCT c.scommentid), 1) AS total_votes
FROM suggestions AS s
LEFT JOIN suggestion_comments AS c ON s.suggestion_id = c.suggestion_id
LEFT JOIN suggestion_votes AS v ON s.suggestion_id = v.suggestion_id
GROUP BY s.suggestion_id
ORDER BY total_votes DESC
LIMIT 30
