From 2003 to 2004, how many customers have paid more than three times?
paymentdate BETWEEN '2003-01-01' AND '2004-12-31'; customers who have paid more than three times refers to (COUNT(customernumber)>3);
SELECT COUNT(customernumber) FROM ( SELECT customernumber FROM payments WHERE STRFTIME('%Y', paymentDate) >= '2003' AND STRFTIME('%Y', paymentDate) <= '2004' GROUP BY customernumber HAVING COUNT(customernumber) > 3 ) T