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CREATE TABLE Articles ( article_id serial PRIMARY KEY ); CREATE TABLE Keywords ( kid serial PRIMARY KEY, article_id INT NOT NULL, keyword VARCHAR (50) ); INSERT INTO Keywords (article_id, keyword) VALUES (0, 'a'), (0, 'b'), (0, 'c'), (1, 'a'), (1, 'd'), (2, 'a'), (2, 'b'), (2, 'c'), (3, NULL); INSERT INTO Articles (article_id) VALUES (0), (1), (2), (3); WITH id_combs AS (SELECT id1, id2 FROM (SELECT A.article_id AS id1, B.article_id AS id2 FROM Articles A, Articles B) AS T WHERE id1 != id2) (SELECT id1, id2 FROM id_combs WHERE NOT EXISTS( ((SELECT K.keyword FROM Keywords K WHERE K.article_id = id1) EXCEPT (SELECT K.keyword FROM Keywords K WHERE K.article_id = id2))) AND NOT EXISTS( ((SELECT K.keyword FROM Keywords K WHERE K.article_id = id2) EXCEPT (SELECT K.keyword FROM Keywords K WHERE K.article_id = id1)))) CROSS JOIN (SELECT 1) AS Y