SELECT (SUM(rank)::float / COUNT(id)::float)::float,
	(	
		SELECT COUNT(DISTINCT(_blog.id))
		FROM   _relation
		JOIN   _blog
				ON _blog.id = _relation.destination_id
		WHERE _relation.path <@ '0.1.40215'
	)::float AS nb_blogs,
	(4 * nb_blogs + 100 * nb_blogs*nb_blogs)/(100 * 1 + 2 * nb_blogs + nb_blogs*nb_blogs)
FROM _top_blog
WHERE id IN (
	SELECT DISTINCT(_blog.id)
	FROM   _relation
	JOIN   _blog
		ON _blog.id = _relation.destination_id
	JOIN   _article
		ON _article.path <@ _blog.path
	WHERE _relation.path <@ '0.1.40215'
	AND    extract (day FROM (localtimestamp - _article.date_publishing))::int < 30
	);

----------------------------------------- 

ERROR:  column "nb_blogs" does not exist