MySQL - Count a GroupBy

Můžete sjednotit všechny hodnoty data a poté je seskupit a spočítat:

SELECT   DATE_FORMAT(d, '%y-%m'), COUNT(*)
FROM     (SELECT date_1 AS d FROM mytable 
          UNION ALL
          SELECT date_2 FROM mytable) t
GROUP BY DATE_FORMAT(d, '%y-%m')
ORDER BY d DESC