Annotating rows if backwards relationship exists (Postgres) - sql

Question

So I have 3 tables: Recommendation, Article and User.
Recommendation has 4 columns:
id | integer
article_id |integer
user_id |integer
submit_time |integer
Article has 3 columns:
id | integer
title
url
I need to obtain a list of all articles, while also annotating each row with a new recommended column, which is 1 if the user in question has recommended the article or 0 if not. There shouldn't be any duplicate Article in the result, and I need it ordered by the Recommendation's submit_time column.
This is on Postgres - 9.1.8.
SELECT DISTINCT ON(t.title) t.title,
t.id, t.url,
MAX(recommended) as recommended
FROM (
SELECT submitter_article.title as title,
submitter_article.id as id,
submitter_article.url as url,
1 as recommended
FROM submitter_article, submitter_recommendation
WHERE submitter_recommendation.user_id=?
AND submitter_recommendation.article_id=submitter_article.id
UNION ALL
SELECT submitter_article.title as title,
submitter_article.id as id,
submitter_article.url as url,
0 as recommended
FROM submitter_article
) as t
GROUP BY t.title, t.id, t.url, recommended
And I'm passing a user id into the ?
I've been trying to do this for a while but can't figure it out. The queries I come up with either return all recommended values as 0, or return duplicate Article rows (one with recommended=0 and the other with recommended=1).
Any ideas?

Answer 1

You don't need a subquery, CASE will do, DISTINCT ON is useless if you also use GROUP BY and you should use explicit joins instead of implicit joins. This query should get you started:
SELECT DISTINCT ON (sa.title) sa.title, sa.id, sa.url,
(CASE
WHEN sr.id IS NULL THEN 0
ELSE 1
END) AS recommended
FROM submitter_article AS sa
LEFT JOIN submitter_recommendation AS sr ON sa.id=sr.article_id
AND sr.user_id=?
ORDER BY sa.title,sr.submit_time DESC;
But there are still some things I'm not sure. You can have two articles with the same title but diffrent id? In that case you can select that which has earlier/later recommendation submit_time but what if there are no recommendations? You need logic for how to select distinct rows and for how to order things in the end.