Stimmt 
Ich hatte etwas Mühe damit, habe aber einen Weg gefunden, es zum Laufen zu bringen. Also, hier ist es!
-- [params]
-- int :months_ago = 0
WITH query_period AS (
SELECT
date_trunc('month', CURRENT_DATE) - INTERVAL ':months_ago months' as period_start,
date_trunc('month', CURRENT_DATE) - INTERVAL ':months_ago months' + INTERVAL '1 month' - INTERVAL '1 second' as period_end
)
SELECT
t.category_id,
SUM (CASE WHEN (u.admin = 'false' OR u.moderator = 'false') THEN 1 ELSE 0 END) AS "Von Mitgliedern gelöst",
SUM (CASE WHEN (u.admin = 't' OR u.moderator = 't') THEN 1 ELSE 0 END) AS "Von Mitarbeitern gelöst"
FROM user_actions ua
JOIN topics t
ON t.id = ua.target_topic_id
JOIN query_period qp
ON ua.created_at >= qp.period_start
AND ua.created_at <= qp.period_end
JOIN users u
ON u.id = ua.user_id
WHERE t.category_id = ANY ('{[LISTE DER KATEGORIE-IDs]}'::int[])
AND ua.action_type = 15
GROUP BY t.category_id ORDER BY t.category_id ASC