MySQL - Count en GroupBy

U kunt alle datumwaarden samenvoegen en ze vervolgens groeperen en tellen:

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