Question

We’re currently stuck trying to make a simple (?) SQL query.

We have these two tables:

create table `post` (
    `id` integer primary key
)
create table `vote` (
    `id` integer primary key,
    `post_id` references `post`.`id`, // Well ok, it's a foreign key then...
    `value` int // 1 for a positive vote, or zero for a negative one
)

We’re trying to build a select that would, for each post, return the number of positive and negative votes: SELECT post.id, <positive count>, <negative count> ...

While this is not hard to do with sub-selects, the challenge begins when we try to do this without sub-selects, but with join. We’re using left outer join, and the problems arise when a post has only positive or negative votes.

While I understand the problem, I can’t figure out how to do that only with join, yet I’m confident it can be done without sub-selects. How would you do that ?

(Well, I did not include my current query, so that it won’t guide you to the wrong direction…)

Answer 1

Previous answer was rubbish, I forgot to GROUP. Here’s an alternative that uses CASE, I THINK it handles the NULL case (since the WHEN clause will be NULL) but you may need to use COALESCE after all…:

SELECT post.id, SUM(CASE WHEN a.vote > 0 THEN 1 ELSE 0 END) up, SUM(CASE WHEN a.vote < 0 THEN 1 ELSE 0 END) down FROM post LEFT JOIN vote a ON (a.post_id = post.id) GROUP BY post.id

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Previous (incorrect) answer:

It sounds like you need to use COALESCE to force a NULL to zero – the following might work but is untested:

SELECT post.id, SUM(COALESCE(a.vote,0)), SUM(COALESCE(b.vote,0)) FROM post LEFT JOIN vote a ON (a.post_id = post.id AND value > 0) LEFT JOIN vote b ON (b.post_id = post.id AND value < 0)