PostgreSQL - GROUP BY clause

Question

I want to search by tags, and then list all articles with that tag, and also how many of given tags they match. So for example I might have:

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Answer 1

When you use a "GROUP BY" clause, you need to enclose all columns that are not grouped in an aggregate function. Try adding title to the GROUP BY list, or selecting "min(a.title)" instead.

SELECT COUNT(t.tag), a.title FROM a_tags t
JOIN w_articles2tag a2t ON a2t.tag = t.id 
JOIN w_article a ON a.id = a2t.article 
WHERE t.tag = 'css' OR t.tag = 'php' GROUP BY t.tag, a.title LIMIT 9