Calculate the number of customers who did not respond in February of 2007.
did not respond refers to RESPONSE = 'false'; February of 2007 refers to REF_DATE BETWEEN '2007-02-01 12:00:00.0'AND '2007-02-28 12:00:00.0';
SELECT COUNT(REFID) custmoer_number FROM Mailings1_2 WHERE RESPONSE = 'false' AND REF_DATE BETWEEN '2007-02-01' AND '2007-02-28'