Rakos, kaut ko samuhļīju no paraugiem, bet nu tas nav vienkāršākais kvērijs:
SELECT ac.id AS category_id, a.id AS article_id, a.heading, a.category AS article_category
FROM article_categories ac
JOIN articles a ON ac.id = a.category
LEFT JOIN articles a2 ON a2.category = ac.id
AND a2.id < a.id
GROUP BY ac.id, ac.name, a.id, a.heading
HAVING COUNT( a2.id ) <2
ORDER BY ac.name, a.heading;