orderId | creationDate
58111   | 2017-01-01 00:00:00
58111   | 2017-01-12 00:00:00
58232   | 2017-01-31 00:00:00
62882   | 2017-02-21 00:00:00
90299   | 2017-03-20 00:00:00

creationMonth | count_orderId
January       | 2
February      | 1
March         | 1

select count( distinct orderId ), year( creationDate ), month( creationDate )
from my_table group by year( creationDate ), month( creationDate )