特定の期間の前後に存在する値をカウントします

それをselectに追加するだけです ：

SELECT o.Customer,
       SUM( o.Order_Date >= '2019-02-01' AND o.Order_Date < '2019-03-01' ) as num_feb_orders
FROM ORDERS o
GROUP BY o.Customer
HAVING SUM( o.Order_Date >= '2019-02-01' AND o.Order_Date < '2019-03-01' ) > 0 AND
       SUM( o.Order_Date >= '2018-01-02' AND o.Order_Date < '2019-02-01' ) = 0 AND
       SUM( o.Order_Date < '2018-01-02' ) > 0 ;