Question: What is the average width and height of the objects in image ID 47? List their object classes as well.
SQL: SELECT T2.OBJ_CLASS, AVG(T1.W), AVG(T1.H) FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T1.IMG_ID = 47 GROUP BY T2.OBJ_CLASS