MYSQL LEFT JOIN returns unexpected results - mysql

Question

I have two tables talk_comments and talk_comment_votes.
I run the following code to select, commentId, numberOfUpvotes, whetherUserUpvoted, numberOfDownvotes, whetherUserDownvoted usin LEFT JOINs to the same table.
SELECT c.id, COUNT(v1.id) as upvotes, COUNT(v2.id) as userUpvoted, COUNT(v3.id) as downvotes, COUNT(v4.id) as userDownvoted FROM talk_comments c
LEFT JOIN talk_comment_votes v1 ON v1.comment_id = c.id AND v1.status = 1
LEFT JOIN talk_comment_votes v2 ON v2.comment_id = c.id AND v2.status = 1 AND v2.user_id = 1 AND v2.is_anonymous = 0
LEFT JOIN talk_comment_votes v3 ON c.id = v3.comment_id AND v3.status = 2
LEFT JOIN talk_comment_votes v4 ON c.id = v4.comment_id AND v4.status = 2 AND v4.user_id = 1 AND v4.is_anonymous = 0
WHERE c.id = 2 GROUP BY c.id
I have the following data in my talk_comment_votes table
So, according to the query, it should select values 2,2,0,1,1 respectively. When I break those JOIN statements and do the queries, it returns the expected results. But, with JOINs, it returns something like the follows.
Can I get some help on fixing this?
Thanks.

Answer 1

I ran a benchmark on queries based on #spencer7593 and #RaymondNijland's 2 answers.
LEFT JOINS wins!
1. Using LEFT JOINS
SELECT c.id, COUNT(DISTINCT v1.id) as upvotes, COUNT(DISTINCT v2.id) as userUpvoted, COUNT(DISTINCT v3.id) as downvotes, COUNT(DISTINCT v4.id) as userDownvoted FROM talk_comments c
LEFT JOIN talk_comment_votes v1 ON v1.comment_id = c.id AND v1.status = 1
LEFT JOIN talk_comment_votes v2 ON v2.comment_id = c.id AND v2.status = 1 AND v2.user_id = 1 AND v2.is_anonymous = 0
LEFT JOIN talk_comment_votes v3 ON c.id = v3.comment_id AND v3.status = 2
LEFT JOIN talk_comment_votes v4 ON c.id = v4.comment_id AND v4.status = 2 AND v4.user_id = 1 AND v4.is_anonymous = 0
WHERE c.id = 2 GROUP BY c.id
Time for 1000 queries: 0.55000805854797s
2. Using Sub Queries
SELECT c.id,c.user_id, c.time,c.body, c.reply_to,
(SELECT COUNT(v1.id) FROM talk_comment_votes v1 WHERE v1.comment_id = c.id AND v1.status = 1 LIMIT 1) as upvotes,
(SELECT COUNT(v2.id) FROM talk_comment_votes v2 WHERE v2.comment_id = c.id AND v2.status = 1 AND v2.user_id = 1 LIMIT 1) as clientUpvoted,
(SELECT COUNT(v3.id) FROM talk_comment_votes v3 WHERE v3.comment_id = c.id AND v3.status = 2 LIMIT 1) as downvotes,
(SELECT COUNT(v4.id) FROM talk_comment_votes v4 WHERE v4.comment_id = c.id AND v4.status = 2 AND v4.user_id = 1 LIMIT 1) as clientDownvoted
FROM talk_comments c
WHERE c.id = 2 GROUP BY c.id
Time for 1000 queries: 0.95499300956726s
3. Using SUM, IF
SELECT c.id
, SUM(IF(v.status = 1 ,1,0)) AS upvotes
, SUM(IF(v.status = 1 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userUpvoted
, SUM(IF(v.status = 2 ,1,0)) AS downvotes
, SUM(IF(v.status = 2 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userDownvoted
FROM talk_comments c
LEFT
JOIN talk_comment_votes v
ON v.comment_id = c.id
WHERE c.id = 2
GROUP BY c.id
Time for 1000 queries: 1.2266919612885s
Thank you for all the answers.

Answer 2

I'd use conditional aggregation. A join to a single reference to tall_comment_votes, and then check conditions in expressions.
SELECT c.id
, SUM(IF(v.status = 1 ,1,0)) AS upvotes
, SUM(IF(v.status = 1 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userUpvoted
, SUM(IF(v.status = 2 ,1,0)) AS downvotes
, SUM(IF(v.status = 2 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userDownvoted
FROM talk_comments c
LEFT
JOIN talk_comment_votes v
ON v.comment_id = c.id
WHERE c.id = 2
GROUP
BY c.id
This avoids the problem of the partial cross product, when there are multiple rows returned from v1, v2, v3 and v4.
The MySQL IF() expression could replaced with a more ANSI standards compliant CASE expression, e.g.
, SUM(CASE WHEN v.status = 1 THEN 1 ELSE 0 END) AS upvotes
FOLLOWUP
setup test case and observe execution plans and performance
populate tables
CREATE TABLE talk_comments (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT);
CREATE TABLE talk_comment_votes (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, comment_id INT UNSIGNED NOT NULL, user_id INT UNSIGNED NOT NULL, is_anonymous TINYINT(1) UNSIGNED NOT NULL, STATUS TINYINT UNSIGNED, time_ INT UNSIGNED);
CREATE INDEX talk_comment_votes_IX1 ON talk_comment_votes (comment_id, STATUS, user_id, is_anonymous) ;
INSERT INTO talk_comments (id) VALUES (1),(2),(3);
INSERT INTO talk_comment_votes (id, comment_id, user_id, is_anonymous, STATUS, time_) VALUES (1,2,2,0,1,0),(2,1,1,0,1,0),(3,2,1,0,2,NULL),(4,7,1,0,2,NULL),(5,1,14,1,1,NULL),(6,2,14,1,1,NULL);
query execution plans
EXPLAIN
SELECT c.id, COUNT(DISTINCT v1.id) AS upvotes, COUNT(DISTINCT v2.id) AS userUpvoted, COUNT(DISTINCT v3.id) AS downvotes, COUNT(DISTINCT v4.id) AS userDownvoted FROM talk_comments c
LEFT JOIN talk_comment_votes v1 ON v1.comment_id = c.id AND v1.status = 1
LEFT JOIN talk_comment_votes v2 ON v2.comment_id = c.id AND v2.status = 1 AND v2.user_id = 1 AND v2.is_anonymous = 0
LEFT JOIN talk_comment_votes v3 ON c.id = v3.comment_id AND v3.status = 2
LEFT JOIN talk_comment_votes v4 ON c.id = v4.comment_id AND v4.status = 2 AND v4.user_id = 1 AND v4.is_anonymous = 0
WHERE c.id = 2 GROUP BY c.id
;
EXPLAIN
SELECT c.id
, SUM(IF(v.status = 1 ,1,0)) AS upvotes
, SUM(IF(v.status = 1 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userUpvoted
, SUM(IF(v.status = 2 ,1,0)) AS downvotes
, SUM(IF(v.status = 2 AND v.user_id = 1 AND v.is_anonymous = 0 ,1,0)) AS userDownvoted
FROM talk_comments c
LEFT
JOIN talk_comment_votes v
ON v.comment_id = c.id
WHERE c.id = 2
GROUP BY c.id
;
output from explain
-- id select_type table type possible_keys key key_len ref rows Extra
-- ------ ----------- ------ ------ ---------------------- ---------------------- ------- ----------------------- ------ -------------
-- 1 SIMPLE c const PRIMARY PRIMARY 4 const 1 Using index
-- 1 SIMPLE v1 ref talk_comment_votes_IX1 talk_comment_votes_IX1 6 const,const 2 Using index
-- 1 SIMPLE v2 ref talk_comment_votes_IX1 talk_comment_votes_IX1 11 const,const,const,const 1 Using index
-- 1 SIMPLE v3 ref talk_comment_votes_IX1 talk_comment_votes_IX1 6 const,const 1 Using index
-- 1 SIMPLE v4 ref talk_comment_votes_IX1 talk_comment_votes_IX1 11 const,const,const,const 1 Using index
-- id select_type table type possible_keys key key_len ref rows Extra
-- ------ ----------- ------ ------ ---------------------- ---------------------- ------- ------ ------ -------------
-- 1 SIMPLE c const PRIMARY PRIMARY 4 const 1 Using index
-- 1 SIMPLE v ref talk_comment_votes_IX1 talk_comment_votes_IX1 4 const 3 Using index
measured performance:
100 executions round 1 round 2 round 3
------------------------------------ ---------- ---------- ---------
multiple left join, count(distinct 0.123 secs 0.130 secs 0.125 secs
conditional aggregation sum(if 0.113 secs 0.114 secs 0.111 secs