Prøv denne forespørgsel -
(redigeret)
SELECT
t.month,
COUNT(DISTINCT t1.column) t1count,
COUNT(DISTINCT t2.column) t2count,
COUNT(DISTINCT t3.column) t3count
FROM
(SELECT MONTH(datetime) AS month FROM t1
UNION
SELECT MONTH(datetime) AS month FROM t2
UNION
SELECT MONTH(datetime) AS month FROM t3
) t
LEFT JOIN t1
ON t.month = MONTH(t1.datetime)
LEFT JOIN t2
ON t.month = MONTH(t2.datetime)
LEFT JOIN t3
ON t.month = MONTH(t3.datetime)
GROUP BY
month