Try something like this
SELECT customer FROM Table
WHERE purchasedate BETWEEN trunc(SYSDATE,'MM') AND LAST_DAY (TO_DATE (trunc(SYSDATE,'MM')))
AND product = 'ABC'
INTERSECT
SELECT customer FROM table
WHERE purchasedate BETWEEN add_months(trunc(SYSDATE,'MM'),-11) AND trunc(SYSDATE,'MM')-1
AND product = 'ABC'
This will only show a customer that has purchased a particular product in the current month as well as in the last 11 months.
Marked 
