Calculate the total amount paid by Stephanie Mitchell for film rentals in June 2005.
the total amount = sum(amount); in June 2005 refers to payment_date like '2005-06%'
SELECT SUM(T1.amount) FROM payment AS T1 INNER JOIN customer AS T2 ON T1.customer_id = T2.customer_id WHERE T2.first_name = 'STEPHANIE' AND T2.last_name = 'MITCHELL' AND SUBSTR(T1.payment_date, 1, 7) = '2005-06'