Question: Calculate the percentage of "airplane" object class in the table.
SQL: SELECT CAST(SUM(CASE WHEN T2.OBJ_CLASS = 'airplane' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(T2.OBJ_CLASS) FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID