Help with a SQL Query - sql

Question

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?

Answer 1

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.

Answer 2

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