What are the names of the dishes with a stable price that were created in April of 2011?
dishes with a stable price refers to highest_price is null; created in April of 2011 refers to SUBSTR(created_at,1,4) = '2011' and SUBSTR(created_at, 7, 1) = '4';
SELECT T1.name FROM Dish AS T1 INNER JOIN MenuItem AS T2 ON T1.id = T2.dish_id WHERE SUBSTR(T2.created_at, 1, 4) = '2011' AND SUBSTR(T2.created_at, 7, 1) = '4' AND T1.highest_price IS NULL